10 ORGANIC CHEMISTRYFunctional groups and homologous seriesNAMING ORGANIC This may come at the beginning or at HOMOLOGOUS SERIESCOMPOUNDS the end of the name, e.g. The alkanes form a series of compoundsOrganic chemistry is concerned with alkane: only hydrogen (-H) joined all with the general formula CnH2n+2,the compounds of carbon. Since there to chain = -e e.g.are more compounds of carbon alcohol: –OH = -ol methane CH4known than all the other elements put amine: –NH2 = amino-together, it is helpful to have a ethane C2H6 halogenoalkane: -X: chloro-,systematic way of naming them. bromo, or iodo- propane C3H81. Identify the longest carbon chain. O butane C4H10 1 carbon = meth- || 2 carbons = eth- aldehyde: –C–H (on the end of the If one of the hydrogen atoms is removed 3 carbons = prop- chain) = -al what is left is known as an alkyl radical 4 carbons = but- R – (e.g methyl CH3–; ethyl C2H5–). O 5 carbons = pent- || When other atoms or groups are 6 carbons = hex- ketone: – C – (not on the end of attached to an alkyl radical they can 7 carbons = hept- the chain) = -one form a different series of compounds. 8 carbons = oct- These atoms or groups attached are O || known as functional groups and the2. Identify the type of bonding in the carboxylic acid: – C–OH = -oic series formed are all homologous series. chain or ring acid All single bonds in the carbon Homologous series have the same chain = -an- OO general formula with the neighbouring || members of the series differing by –CH2; One double bond in the carbon ester: – C–OR: = -oate chain = -en- for example the general formula of One triple bond in the carbon 4. Numbers are used to give the alcohols is CnH2n+1OH. The chemical chain = -yn- positions of groups or bonds along properties of the individual members of the chain. an homologous series are similar and3. Identify the functional group they show a gradual change in physical joined to the chain or ring. properties.SOME COMMON FUNCTIONAL GROUPS Formula Name Examples R–H alkane H–C–H H– C– C– C– C–H H– C– C– C– H methane butane 2-methylpropane R–OH alcohol H–C–C–O–H H–C–C–C–O–H H–C–C– C– H ethanol propan-1-ol propan-2-ol R–NH2 amine H–C–C–N H–C–C–C–C–H ethylamine (aminoethane) 2-aminobutane Cl Cl Cl H R–X halogenoalkane H – C – C – Br H–C–C–H H–C–C– H (X = F, Cl, Br, or I) Cl bromoethane 1,2-dichloroethane 1,1-dichloroethane O O O R–C–H aldehyde H–C–C H–C–C–C H H ethanal propanal O O O O R–C–R´ ketone H3C – C – CH3 H3C – C – CH2 – CH2– CH3 H3C – CH2 – C – CH2– CH3 (R´ may be the same as or different to R) propanone pentan-2-one pentan-3-one O O O R–C–OH carboxylic acid H–C C2H5 – C O–H O–H methanoic acid propanoic acid O O O R–C–OR´ ester H–C–C–O–C–C–H H – C – O – C3H7 ethyl ethanoate propyl methanoate Organic chemistry 61
Properties of different functional groups Structural isomers BOILING POINTS STRUCTURES OF HYDROCARBONS As the carbon chain gets longer the mass of the molecules increases and the van der Isomers of alkanes H–C–C–C–C–C–H Waals’ forces of attraction increase. A plot of boiling point against number of carbon Each carbon atom contains four single atoms shows a sharp increase at first, as the percentage increase in mass is high, but as b. pt 36.3 °C bonds. There is only one possible successive –CH2– groups are added the rate of increase in boiling point decreases. structure for each of methane, ethane, H and propane however two structures of When branching occurs the molecules become more spherical in shape, which reduces butane are possible. the contact surface area between them and lowers the boiling point. butane 2-methylpropaneOther homologous series show similar trends but the actual temperatures at which the b. pt 27.9 °Ccompounds boil will depend on the types of attractive forces between the molecules. HThe volatility of the compounds also follows the same pattern. The lower members of These are examples of structural isomers.the alkanes are all gases as the attractive forces are weak and the next few members are Structural isomers have the samevolatile liquids. Methanol, the first member of the alcohols is a liquid at room molecular formula but a differenttemperature, due to the presence of hydrogen bonding. Methanol is classed as volatile structural formula. They normally have H b. pt 9.5 °Cas its boiling point is 64.5 °C but when there are four or more carbon atoms in the similar chemical properties but theirchain the boiling points exceed 100 °C and the higher alcohols have low volatility. physical properties may be slightly pentane 2-methylbutane 2,2-dimethylpropaneCompound Formula Mr Functional group Strongest type of attraction B. pt / °C different. There are three structural (b. pt 36.3 °C) (b. pt 27.9 °C) (b. pt 9.5 °C) isomers of pentane.butane C4H10 58 alkane van der Waals’ –0.5butene C4H8 56 alkene van der Waals’ –6.2butyne C4H6 54 alkyne van der Waals’ 8.1 Structures of alkenesmethyl methanoate HCOOCH3 60 ester dipole:dipole 31.5 Ethene and propene only CH3 CH2 – CH3 CH3 CH3 CH3propanal CH3CH2CHO 58 aldehyde dipole:dipole 48.8 have one possible structure CH3propanone CH3COCH3 58 ketone dipole:dipole 56.2 each but butene has three structural isomers. ethene propene but-1-ene but-2-ene 2-methylpropeneaminopropane CH3CH2CH2NH2 59 amine hydrogen bonding 48.6propan-1-ol CH3CH2CH2OH 60 alcohol hydrogen bonding 97.2ethanoic acid CH3COOH 60 carboxylic acid hydrogen bonding 118 CLASSIFICATION OF ALCOHOLS AND NAMING STRUCTURAL ISOMERS HALOGENOALKANES The naming system explained on page 61 is known as the IUPAC Alcohols and halogenoalkanes may be classified (International Union of Pure and Applied Chemistry) system. The SOLUBILITY IN WATER according to how many R- groups and how many IUPAC names to distinguish between structural isomers of alcohols, Whether or not an organic compound will be soluble in water depends on the polarity of the functional group and on the chain hydrogen atoms are bonded to the carbon atom aldehydes, ketones, carboxylic acids and halogenoalkanes length. The lower members of alcohols, amines, aldehydes, ketones, and carboxylic acids are all water soluble. However, as the containing the functional group. containing up to six carbon atoms are required. length of the non-polar hydrocarbon chain increases the solubility in water decreases. For example, ethanol and water mix in all For example, four different structural isomers with the molecular proportions, but hexan-1-ol is only slightly soluble in water. Compounds with non-polar functional groups, such as alkanes, and primary (on R-group bonded to C atom) formula C6H12O are shown. alkenes, do not dissolve in water but are soluble in other non-polar solvents. Propan-1-ol is a good solvent because it contains both polar and non-polar groups and can to some extent dissolve both polar and non-polar substances. H H H R C OH R C Br H C H STRUCTURAL FORMULAS H H H H O The difference between the empirical, molecular and structural formulas of a compound has been covered in Topic 1 - H C C C C quantitative chemistry. Because the physical and chemical properties of organic compounds are determined by the functional secondary (two R-group bonded to C atom) group and the arrangement of carbon atoms within the molecule, the structural formulas for organic compounds are often used. R may be the same as R or different H H H H H O H H H The structural formula unambiguously shows how the atoms are bonded together. All the hydrogen atoms must be shown when H H H C C C C C C H C H drawing organic structures. The skeletal formula showing just the carbon atoms without the hydrogen atoms is not acceptable R C OH R C Br H H H H H H H except for benzene (see below). However, unless specifically asked, Lewis structures showing all the valence electrons are not necessary. The bonding must be clearly indicated. Structures may be shown using lines as bonds or in their shortened form e.g. R R or CH3CH2CH2CH2CH2CHO CH3C(CH3)2CH2CHO CH3CH2CH2CH2CH3 or CH3–(CH2)3–CH3 for pentane but the molecular formula C5H12 will not suffice. hexanal 3,3-dimethylbutanal Tertiary (three R-group bonded to C atom) H H H H H H H R" R" H H H C C C C C C H C C C C C C H C R C OH R C Br H C H H C H H C C H O H H H O H H H H H H H H R R C C H C C C C C H H C C C C C H structural formula of hexane skeletal formula of hexane H C H also acceptable not acceptable as structural formula H H H H H H H H CH3CH2CH2CH2CH2CH3 H or CH3COCH(CH3)CH2CH3 or CH3CH(CH3)COCH2CH3 three different ways of showing the structural formula and CH3(CH2)4CH3 of benzene, all are acceptable 3-methylpentan-2-one 2-methylpentan-3-one62 Organic chemistry Organic chemistry 63
Alkanes AlkenesLOW REACTIVITY OF ALKANES MECHANISM OF CHLORINATION ADDITION REACTIONSBecause of the relatively strong C–C and C–H bonds and OF METHANE The bond enthalpy of the C=C double bond in alkenes has a value of 612 kJ mol–1. This is less than twice the average value ofbecause they have low polarity, alkanes tend to be quite The mechanism of an organic reaction describes the 348 kJ mol–1 for the C–C single bond and accounts for the relative reactivity of alkenes compared to alkanes. The most importantunreactive. They only readily undergo combustion reactions individual steps. When chemical bonds break they may reactions of alkenes are addition reactions. Reactive molecules are able to add across the double bond. The double bond is saidwith oxygen and substitution reactions with halogens in break heterolytically or homolytically. In heterolytic fission to be unsaturated and the product, in which each carbon atom is bonded by four single bonds, is said to be saturated.ultraviolet light. both of the shared electrons go to one of the atoms resulting in a negative and a positive ion. In homolytic fission each of the two atoms forming the bond retains one of the shared C=C + X –Y –C–C– electrons resulting in the formation of two free radicals. The unsaturated saturated bond between two halogen atoms is weaker than the C–HCOMBUSTION or C–C bond in methane and can break homolytically in the Addition reactions include the addition of hydrogen, bromine, hydrogen halides, and water.Alkanes are hydrocarbons - compounds that contain carbon presence of ultraviolet light.and hydrogen only. All hydrocarbons burn in a plentiful Cl2 → Cl• + Cl•supply of oxygen to give carbon dioxide and water. Thegeneral equation for the combustion of any hydrocarbon is: This stage of the mechanism is called initiation. H–C–C–H y y CxHy + (x + )O2 → xCO2 + H2O Free radicals contain an unpaired electron and are highly 4 2 reactive. When the chlorine free radicals come into contactAlthough the C–C and C–H bonds are strong the C=O and with a methane molecule they combine with a hydrogen (alkane)O–H bonds in the products are even stronger so the reaction is atom to produce hydrogen chloride and a methyl radical.very exothermic and much use is made of the alkanes as fuels. H2 H3C–H + Cl• → H3C• + Cl•e.g natural gas (methane) Since a new radical is produced this stage of the mechanismCH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH o = –890.4 kJ mol–1 is called propagation. The methyl free radical is also Br Br Br HBr Br2 extremely reactive and reacts with a chlorine molecule to H–C–C–H C=C H–C–C–Hgasoline (petrol) form the product and regenerate another chlorine radical.C8H18(l) + 121⁄2O2(g) → 8CO2(g) + 9H2O(l) ΔH o = –5512 kJ mol–1 This is a further propagation step and enables a chain reaction to occur as the process can repeat itself. bromoethane H2O 1,2-dibromoethaneIf there is an insufficient supply of oxygen then incomplete (halogenoalkane) (H2SO4 catalyst) (dihalogenoalkane)combustion occurs and carbon monoxide and carbon are also CH3 + Cl2 → CH3–Cl + Cl• •produced as products. In theory a single chlorine radical may cause up to 10 000 OH molecules of chloromethane to be formed. Termination H–C–C–H occurs when two radicals react together. Cl • + Cl • → Cl2 ⎫SUBSTITUTION REACTIONS CH3 + Cl • → CH3Cl • ⎬ termination (alcohol)Alkanes can react with chlorine (or other halogens) in the CH3 + CH3 → C2H6 • • ⎭presence of ultraviolet light to form hydrogen chloride and asubstituted alkane, e.g. methane can react with chlorine to Further substitution can occur when chlorine radicals react with the substituted products. For example: USES OF ADDITION ADDITION POLYMERIZATIONform chloromethane and ethane can react with bromine to REACTIONS Under certain conditions ethene can also undergo addition reactions with itselfform bromoethane. H H 1. Bromination to form a long chain polymer containing many thousands (typically 40 000 to Pure bromine is a red liquid but it has 800 000) of carbon atoms. H H Cl C H + Cl Cl – C + HCl a distinctive yellow/orange colour in UVH C H + Cl – Cl H C Cl + H – Cl H H solution. When a solution of bromine is added to an alkene the product is H H H H colourless. This decolorization of n C=C ( (n – CH2 – CH2 – methane chloromethane bromine solution provides a useful test then Cl C + Cl2 Cl – C – Cl + Cl poly(ethene) to indicate the presence of an alkene (also known as H H H H H H dichloromethane ethene group. polythene) UVH C C H + Br – Br H C C Br + H– Br The substitution can continue even further to produce 2. Hydration trichloromethane and then tetrachloromethane. Ethene is an important product formed These addition reactions can be extended to other substituted alkenes to give a H H H H during the cracking of oil. Although wide variety of different addition polymers. ethane bromoethane The overall mechanism is called free radical substitution. ethanol can be made from the [Note that in this mechanism hydrogen radicals H• are not fermentation of starch and sugars, e.g. H Cl formed.] much industrial ethanol is formed n C=C ( (n – CH2 – CHCl – from the addition of steam to ethene. H H poly(chloroethene) 3. Hydrogenation chloroethene (also known as polyvinylchloride, PVC) The addition of hydrogen to unsaturated vegetable oils is used F F industrially to make margarine. n C=C ( ( – CF2 – CF2 –n Hydrogenation reduces the number of F F double bonds in the polyunsaturated poly(tetrafluoroethene), PTFE vegetable oils present in the tetrafluoroethene (also known as Teﬂon or ‘non-stick’) margarine, which causes it to become a solid at room temperature.64 Organic chemistry Organic chemistry 65
Alcohols Substitution reactions and reaction pathwaysCOMBUSTION SUBSTITUTION REACTIONS OF HALOGENOALKANESEthanol is used both as a solvent and as a fuel. It combusts completely in a plentiful supply of oxygen to give carbon dioxide and Because of the greater electronegativity of the halogen atom compared with the carbon atom halogenoalkanes have a polarwater. bond. Reagents that have a non-bonding pair of electrons are attracted to the carbon atom in halogenoalkanes and a substitution reaction occurs. Such reagents are called nucleophiles C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) ΔH o = –1371 kJ mol–1 A double-headed curly arrow represents the movement of a pairEthanol is already partially oxidized so it releases less energy than burning an alkane of comparable mass. However, it can be of electrons. It shows where they come from and where theyobtained by the fermentation of biomass so in some countries it is mixed with petrol to produce ‘gasohol’ which decreases the Nu– Cδ+ C + Br – move to.dependence on crude oil. Brδ− NuThe general equation for an alcohol combusting completely in oxygen is: CnH(2n+1)OH + (2n–1)O2 → nCO2 + (n+1)H2O MECHANISM OF NUCLEOPHILIC SUBSTITUTION Tertiary halogenoalkanes (three alkyl groups attached to the carbon atom bonded to the halogen) Primary halogenoalkanes (one alkyl group attached to e.g. the reaction between 2-bromo-2-methylpropane andOXIDATION OF ETHANOL the carbon atom bonded to the halogen) warm dilute sodium hydroxide solution.Ethanol can be readily oxidized by warming with an acidified solution of potassium dichromate(VI). During the process the e.g. the reaction between bromoethane and warm diluteorange dichromate(VI) ion Cr2O72– is reduced from an oxidation state of +6 to the green Cr3+ ion. Use is made of this in simple sodium hydroxide solution. C(CH3) Br + OH– C(CH3) OH + Br –breathalyser tests, where a 3 3 C2H5Br + OH– → C2H5OH + Br–motorist who is suspected of H H H O H O The experimentally determined rate expression for this Cr2O72–/H+ Cr2O72–/H+having exceeded the alcohol C C C OH H C C H C C The experimentally determined rate expression is: reaction is: rate = k[C(CH3)3Br]limit blows into a bag containing H H H H H O H rate = k [C2H5Br][OH–]crystals of potassium ethanol ethanol ethanoic acid A two-step mechanism is proposed that is consistent with this (’wine’) (’vinegar’) The proposed mechanism involves the formation of a rate expression.dichromate(VI). transition state which involves both of the reactants.Ethanol is initially oxidized to ethanal. The ethanal is then oxidized further to ethanoic acid. slow H H H C(CH3) Br C(CH3)+ + Br –Unlike ethanol (b. pt 78.5 °C) and ethanoic acid (b. pt 118 °C) ethanal (b. pt 20.8 °C) does not have hydrogen bonding between – 3 3its molecules, and so has a lower boiling point. To stop the reaction at the aldehyde stage the ethanal can be distilled from the HO– Cδ+ HO C Br C + Br –reaction mixture as soon as it is formed. If the complete oxidation to ethanoic acid is required, then the mixture can be heated CH3 Brδ− CH3 H HO CH3 fastunder reflux so that none of the ethanal can escape. H H C(CH3)+ + OH– C(CH3) OH 3 3 In this reaction it is the first step that is the rate determining Because the molecularity of this single-step mechanism is step. The molecularity of this step is one and the mechanism OXIDATION OF ALCOHOLS two it is known as an SN2 mechanism (bimolecular is known as SN1 (unimolecular nucleophilic substitution). Ethanol is a primary alcohol, that is the carbon atom bonded to the –OH group is bonded to two hydrogen atoms and one alkyl nucleophilic substitution). group. The oxidation reactions of alcohols can be used to distinguish between primary, secondary, and tertiary alcohols. The mechanism for the hydrolysis of secondary All primary alcohols are halogenoalkanes (e.g 2-bromopropane CH3CHBrCH3) is H oxidized by acidified potassium Cr2O72–/H+ O Cr2O72–/H+ O more complicated as they can proceed by either SN1 or SN2 R – C – OH R–C R–C dichromate(VI), first to H OH pathways or a combination of both. H aldehydes then to carboxylic primary alcohol aldehyde carboxylic acid acids. REACTION PATHWAYS H dihalogenoalkane trihalogenoalkane Using the scheme on the left which summarizes the organic Secondary alcohols are oxidized O alkane Cr2O72– /H+ R – C – R′ tetrahalogenoalkane reactions in the text, it is possible to devise reaction to ketones, which cannot R – C – OH R′ pathways. These should involve no more than two steps undergo further oxidation. and should include the reagents, conditions and relevant secondary alcohol ketone equations. R halogenoalkane alkene poly(alkene) e.g. to convert but-2-ene to butanone Tertiary alcohols cannot be R′ – C – OH Step 1. Heat but-2-ene in the presence of H2SO4 as a oxidized by acidified R″ catalyst to form butan-2-ol dichromate(VI) ions as they have tertiary alcohol H OH no hydrogen atoms attached H H H2SO4 directly to the carbon atom containing the –OH group. It is not true to say that tertiary alcohols can never be oxidized, as they alcohol aldehyde carboxylic acid C C + H2O H3C C C CH3 burn readily, but when this happens the carbon chain is destroyed. H3C CH3 H H but-2-ene butan-2-ol Step 2. Oxidize butan-2-ol to but-2-ene by warming with ketone acidified potassium dichromate(VI) solution H OH H O H+/Cr2O72- H3C C C CH3 H3C C C CH3 H H H butan-2-one66 Organic chemistry Organic chemistry 67
Identifying and naming more functional groups Nucleophilic substitution AMINES (R-NH2) ESTERS (R-COO-R’) NUCLEOPHILIC SUBSTITUTION IUPAC accepts several different ways of naming amines. Esters take their IUPAC name from the acid and alcohol The reaction between halogenoalkanes and a warm dilute aqueous solution of sodium hydroxide is a The most straightforward system is to prefix the longest chain from which they are derived. The first part of the ester is nucleophilic substitution reaction. Other nucleophiles are CN–, NH3 and H2O. The nucleophiles are alkane by the word amino- with the location of the NH2– named after the R- group from the alcohol. There is then a attracted to the δ+ carbon atom and substitute the halogen atom in halogenoalkanes. group being indicated. For example, 2-aminopentane and 1- space followed by the name for the carboxylic acid anion. Primary halogenoalkanes react by an SN2 mechanism: aminohexane. It is also correct to call them by the longest For example, methyl ethanoate, ethyl propanoate and alkane with the suffix –amine e.g. pentan-2-amine. If the propyl methanoate. C2H5Br + OH– → C2H5OH + Br – number of carbon atoms is small (one, two or three) then the H O H and tertiary halogenoalkanes react by an SN1 mechanism. old names of methylamine, ethylamine and propylamine CH3CH2COOCH2CH3 tend to be used rather than aminomethane, aminoethane H C C O C H C(CH3)3 Br + OH– ⎯⎯→ C(CH3)3 OH + Br – and aminopropane. IUPAC accepts 1-butylamine, 1- H H ethyl propanoate There are several factors which affect the rate of the substitution reactions. butanamine and 1-aminobutane for CH3CH2CH2CH2NH2. O methyl ethanoate H NH2 H H H H H H H H H H C O CH2 CH2 CH3H C C C C C H H C C C C C C NH2 FACTORS AFFECTING THE RATE OF NUCLEOPHILIC SUBSTITUTION propyl methanoate H H H H H H H H H H H CH3CH(NH2)CH2CH2CH3 CH3(CH2)5NH2 THE NATURE OF THE THE NATURE OF THE THE NATURE OF THE 2-aminopentane 1-aminohexane NITRILES (R-CN) (or pentan-2-amine ) (or hexan-1-amine ) Nitriles used to be called cyanides so that C2H5CN was NUCLEOPHILE HALOGEN HALOGENOALKANE known as ethyl cyanide. IUPAC bases the name on the The effectiveness of a nucleophile For both SN1 and SN2 reactions the Tertiary halogenoalkanes react faster For secondary amines the main name of the amine is taken depends on its electron density. iodoalkanes react faster than than secondary halogenoalkanes, from the longest carbon chain attached to the nitrogen atom. longest carbon chain (which includes the carbon atom of the nitrile group) with the word –nitrile is added to the Anions tend to be more reactive than bromoalkanes, which in turn react which in turn react faster than The other chain is prefixed as an alkyl group with the the corresponding neutral species. For faster than chloroalkanes. This is primary halogenoalkanes. The SN1 location prefix given as an italic N. Examples include alkane. For example, the IUPAC name for C2H5CN is propanenitrile. Ethanenitrile has the formula CH3CN, and example, the rate of substitution with due to the relative bond energies, route, which involves the formation N-methylethanamine and N-ethylpropanamine. Tertiary the hydroxide ion is faster than with as the C–I bond is much weaker of an intermediate carbocation, is amines conatin two prefixes with an italic N, for example butanenitrile the formula C3H7CN. water. Among species with the same than the C–Cl bond and therefore faster than the SN2 route, which CH3CH2N(CH3)2 is N,N-dimethylethanamine. H H H H H H charge a less electronegative atom breaks more readily. involves a transition state with a H C C C N H C C N H C C C C N carrying a non bonded pair of relatively high activation energy. H H CH3 Bond enthalpy / kJ mol –1 electrons is a better nucleophile than N N N H H H H H H a more electronegative one. Thus C–I 238 H3C C2H5 C2H5 C3H7 H3C C2H5 propanenitrile ethanenitrile butanenitrile ammonia is a better nucleophile than C–Br 276 water. This is because the less N-methylethanamine N-ethylpropanamine N,N-dimethylethanamine electronegative atom can more easily C–Cl 338 donate its pair of electrons as they are held less strongly. CN– > OH– > NH3 > H2O AMIDES (R-CO-NH2) order of reactivity of common Amides are named after the longest carbon chain (which nucleophiles includes the carbon atom in the functional group) followed by –amide. For example, ethanamide and 2-methylpropanamide. Secondary amides are named rather like amines in that the other alkyl group attached to the In addition to forming alcohols when water or hydroxide The nucleophilic substitution reactions of halogenoalkanes nitrogen atom is prefixed by an N, e.g., N-methylethanamide ions are used as the nucleophile, halogenoalkanes can react makes them particularly useful in organic synthesis. The O H H O O with ammonia to form amines and with cyanide ions to form reaction with potassium cyanide provides a useful means of CH3 nitriles. With primary halogenoalkanes the mechanism is increasing the length of the carbon chain by one carbon CH3 C NH2 H C C C NH2 CH3 C N SN2 in both cases, e.g. with bromoethane and cyanide ions atom. The nitrile can then be converted either into amines H propanenitrile is produced. by reduction using hydrogen with a nickel catalyst or into H H H carboxylic acids by acid hydrolysis, e.g. H C H – H – H2 / Ni H NC C NC C Br C + Br – CH3CH2CH2NH2 H Br H propanamine ( propylamine) ethanamide 2-methylpropanamide N-methylethanamide CH3 CH3 NC CH3 CH3CH2CN H H+ / H2O When bromoethane reacts with ammonia, ethylamine is CH3CH2COOH + NH4+ produced. However ethylamine also contains a nitrogen propanoic acid atom with a non- bonding pair of electrons so this too can act as a nucleophile and secondary and tertiary amines can be formed. Even the tertiary amine is still a nucleophile and can react further to form the quaternary salt. C2H5Br C2H5Br C2H5Br C2H5Br NH3 C2H5NH2 (C2H5)2NH (C2H5)3N (C2H5)4N+Br– + HBr + HBr + HBr68 Organic chemistry Organic chemistry 69
Elimination and condensation reactions Condensation polymerization and reaction pathways ELIMINATION REACTIONS OF HALOGENOALKANES CONDENSATION POLYMERIZATION The reactions of halogenoalkanes with hydroxide ions provide an example of how altering the reaction conditions can cause Condensation involves the reaction between two molecules to eliminate a smaller molecule, such as water or hydrogen the same reactants to produce completely different products. (Note that another good example is the reaction of chloride. If each of the reacting molecules contain two functional groups that can undergo condensation, then the methylbenzene with chlorine.) With dilute sodium hydroxide solution the OH– ion acts as a nucleophile and substitution condensation can continue to form a polymer. occurs to produce an alcohol, e.g. An example of a polyester is polyethene terephthalate (known as Terylene in the UK and as Dacron in the USA) used for textiles, which is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. HO–: R–Br ⎯→ R–OH + Br – O O O O n HO – C – C – OH + n H – O – CH2 – CH2 – OH HO C C – O – CH2 – CH2 – O H + (2n–l) H2O However with hot alcoholic sodium hydroxide solution (i.e. sodium hydroxide dissolved in ethanol) elimination occurs and n benzene-1, 4-dicarboxylic acid ethane-1, 2-diol repeating unit ‘Terylene’ or ‘Dacron’ an alkene is formed, e.g. H Br Amines can also condense with carboxylic acids to form an amide link (also known as a peptide bond). One of the best known examples of a polyamide is nylon. C C + OH– ⎯→ C=C + H2O + Br – amide link O O O O H H H H In this reaction the hydroxide ion reacts as a base. The elimination of HBr can proceed either by a carbocation or as a n HO – C – CH2– C – OH + n N – CH2 – N HO C CH2 – C – N – (CH2)6 – N + (2n–l) H2O 4 6 nH concerted process, e.g. H H 4 H H H H H H hexane-1, 6-dioic acid 1,6-diaminohexane repeating unit nylon 6,6 – – – – – – H–C–C–C–H H – C – C – C – H + Br– (6, 6 because each monomer contains 6 carbon atoms) + – – – – – H Br H H H : OH– H– H C=C– – + H2O REACTION PATHWAYS – H CH3 The compounds and reaction types covered in the AHL can be summarized in the following scheme: or – HO: alkene halogenoalkane nitrile H H H H– H – – – – – H–C–C–C–H C=C– + H2O + Br – – – – H CH3 H Br H In the presence of ethanol there will also be some ethoxide ions present. Ethoxide is a stronger base than hydroxide so the equilibrium lies to the left but some ethoxide ions will be present and these may be the actual species acting as the base. alcohol amine HO– + C2H5OH H2O + C2H5O– ester carboxylic acid amide CONDENSATION REACTIONS Given the starting materials, two step syntheses for new products can be devised. For example, the conversion of A condensation reaction involves the reaction between two molecules to produce a larger molecule with the elimination of a 1-bromopropane to 1-aminobutane (1-butylamine) can be performed in the following two stages. small molecule such as water or hydrogen chloride. One important condensation reaction is the formation of esters when an alcohol reacts with a carboxylic acid. Step 1. 1-bromopropane can undergo nucleophilic substitution with potassium cyanide solution to form propanenitrile. H O H H H O H H CH3CH2CH2Br + CN– ⎯→ CH3CH2CH2 CN + Br – e.g. H C C OH + H O C C H H C C O C C H + H2O H H H H H H Step 2. Propanenitrile can be reduced by heating with hydrogen over a nickel catalyst. ethanoic acid ethanol ethyl ethanoate Ni CH CH CH CN + 2H ⎯→ CH CH CH CH NH 3 2 2 2 3 2 2 2 2 Most esters have a distinctive, pleasant, fruity smell and are used both as natural and artificial flavouring agents in food. For example, ethyl methanoate HCOOCH2CH3 is added to chocolate to give it the characteristic flavour of ‘rum truffle’. Esters are Another example would be the formation of ethylamine starting with ethane. Now reactions covered in the core can also be also used as solvents in perfumes and as plasticizers (substances used to modify the properties of polymers by making them included. more flexible). Step 1. React ethane with chlorine in ultraviolet light so that chloroethane is formed by free radical substitution. Another example of a condensation reaction is the formation of secondary amides by reacting a carboxylic acid with an amine. uv C H + Cl ⎯→ C H Cl + HCl 2 6 2 2 5 O H O R C OH + H N R R C N R + H2O Step 2. React chloroethane with ammonia. amide C2H5Cl + NH3 ⎯→ C2H5NH2 + HCl This reaction is important in biological reactions as amino acids contain an amine group and a carboxylic acid group so that amino acids can condense together in the presence of enzymes to form poly(amides).70 Organic chemistry Organic chemistry 71
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