Unit 2 8 Alcohols And Halogenoalkanes Notes


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The notes to accompany unit 2.8 'Organic chemistry - alcohols and halogenoalkanes'

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Unit 2 8 Alcohols And Halogenoalkanes Notes

  1. 1. Unit 2.8 - Organic chemistry - alcohols and halogenoalkanes Revision - IUPAC system for naming organic compounds You should remember from unit 1 that there is a set of rules, set by IUPAC, for classifying and naming organic compounds. As we will be studying further organic chemistry in this unit now is a good time to remind yourself of those rules: Number of Position of functional groups in the chain Prefix Carbons in chain 1. The 'parent' chain is the ___________ possible chain meth- 2. The carbon with the functional group attached is given eth- a ____________. prop- 3. Numbering starts from the end of the chain that gives the carbon being considered the ____________ number. but- pent- Alkyl groups hex- methyl hept- 1. Position in chain is ethyl indicated by oct- ___________ possible propyl number. dec- 2. If more than 1 side chain is attached then the name dodec- of the compound includes the side groups in _________________ order, regardless of which eicos- carbon atom they are found on. Try these examples: 4-methylpent-1-ene E-hex-2-ene 1. Alcohols Introduction Ethanol is the compound generally referred to as alcohol and found in alcoholic drinks. It is most commonly produced through the action of yeast
  2. 2. on natural sugars otherwise known as fermentation. Many other alcohols do exist; in chemistry ‘alcohol’ refers to the homologous series of molecules containing the hydroxyl (- OH) functional group. The reactivity of alcohols is based on the hydroxyl group. They are industrially important as raw materials in the synthesis of other compounds and as solvents. Naming Naming alcohols is relatively simple - the name of the alkyl group containing the ______________ number of carbons is used and the suffix ______ is added. Here are some examples: Name Displayed formula Skeletal formula Methanol Ethanol Propan-1-ol ________pentan-2- ol Primary, secondary or tertiary? There are three main types of alcohol - primary , secondary and tertiary ; they are defined by the number of alkyl (- C) groups attached to the carbon with the hydroxyl (-OH) group: Primary alcohols Ethanol Primary alcohols have the general structure RCH2OH. There are two hydrogens and one alkyl group attached to the same carbon as the hydroxyl group
  3. 3. Secondary alcohols Propan-2-ol Secondary alcohols have the general structure RR1CHOH. There is one hydrogen and two __________ groups attached to the same carbon as the hydroxyl group Tertiary alcohols 2-methylpropan-2-ol Tertiary alcohols… NB These differences in structure are very important as they affect the way that the alcohol reacts! Reactions of Alcohols - combustion, reduction, oxidation and substitution a. Combustion Alcohols burn readily in air or even more readily in pure oxygen. Many burn with an almost unperceivable blue flame. Combustion of alcohols produces carbon dioxide and _________ CH3CH2OH(l) + ___O2(g) → ___CO2(g) + ___H2O(l) CH3CH2CH2CH2OH(l) + ___O2(g) → ___CO2(g) + ___H2O(l) b. Reaction with sodium • Sodium reacts vigorously with water to produce _________________ and ____________. The sodium is less dense than water so it floats, fizzes and moves around. Water acts as an acid in this reaction by losing a proton • The reaction with alcohols is _________vigorous and the sodium _________ in the ethanol as it is _________ dense than the alcohol 2Na(s) + 2CH3CH2OH(l) → 2CH3CH2O-Na+(alc) + H2(g) sodium ethoxide • The products are _________________ and ___________, the reaction is less vigorous because ethanol is a _____________ acid than water
  4. 4. c. Oxidation Oxidation reactions are a useful test to find out if an alcohol is primary, secondary or tertiary as they each give different products. Common oxidising agents such as acidified potassium dichromate(VI) and potassium manganate(VII) can be used… • Primary alcohols - Are readily oxidised to form aldehydes, which then rapidly oxidise to form carboxylic acids: + H2O Propan-1-ol Propanal Propanoic acid Primary alcohol Aldehyde Carboxylic acid* *The most common product of this oxidation is the carboxylic acid; however, the aldehyde can be separated during the reaction • Secondary alcohols - Are readily oxidised to form ketones with no further oxidation occurring: Propan-2-ol Propanone Secondary alcohol Ketone • Tertiary alcohols - Are not oxidised by any of the common oxidising reactions, there is simply no reaction Oxidation as a test for primary, secondary or tertiary Add acidified potassium dichromate (VI) Primary - Secondary - Tertiary - Chromium(III) compounds are dark green
  5. 5. Distillation - separating the products of oxidation • Distillation can be used to prepare a ketone from a secondary alcohol: Compound Boiling temp (°C) Butanone 80 Butan-2-ol 95 Only the ketone will be collected if the temperature is kept between 80 °C & 94 °C • Or to separate the products of oxidation of a primary alcohol: Compound Boiling temp (°C) Ethanol 78 Ethanal 21 Ethanoic acid 118 The aldehyde can be collected between 21 °C & 70 °C Reflux - preparing carboxylic acids from primary alcohols Heating under reflux ensures that the oxidation reaction is completed Any aldehyde or unreacted primary alcohol evaporates, condenses here in the _______________ and falls back into the reaction vessel where it can it oxidised The oxidation reaction takes place here. First the aldehyde is produced then, after further oxidation, the ________________. Heat causes the products to _____________. d. Substitution to form halogenoalkanes Alcohols react with phosphorous(V) chloride (PCl5) to produce a chloroalkane and hydrogen chloride gas… CH3CH2OH(l) + PCl5(s) → CH3CH2Cl(l) + HCl(g) NB this reaction can be used as a test for alcohols - if you add PCl5 to an unknown liquid the production of HCl gas is evidence for the presence of an -OH group. Preparation - halogenoalkanes from alcohols e.g. the preparation of 1-bromobutane from butan-1-ol…
  6. 6. Step 1 - Sodium bromide reacts with conc. sulphuric to produce hydrogen bromide NaBr(s) + H2SO4(l) → NaHSO4 + HBr(g) Step 2 - Hydrogen bromide reacts with alcohol to produce 1-bromobutane CH3CH2CH2CH2OH(l) + HBr(g) → CH3CH2CH2CH2Br(l) + H2O9(l) Step 3 - The products are distilled to yield a mixture, which separates into 2 layers - aqueous and organic, a separating funnel is used to collect the organic layer Step 4 - The aqueous layer is discarded and the organic layer is redistilled to produce 1- bromobutane, which has a boiling temperature of 102 °C 2. Halogenoalkanes Introduction Halogenoalkanes are saturated hydrocarbons with one or several halogen functional groups. The halogens are found in group ___ and consist of __________, ___________, __________ and ____________. Uses of halogenoalkanes (p214- p217) Until 1986 chlorofluorocarbons (CFCs) were routinely used as refrigerants in fridges and freezer and propellants in aerosols; however, it was discovered that they undergo radical reactions when exposed to UV radiation high in the atmosphere, which lead to the removal of ozone (O 3 ) from the ozone layer… CCl2F2 → •CClF2 + Cl• Cl• + O3 → ClO• + O2 ClO• + O3 → Cl• + 2O2 Net reaction: 2O 3 → 3O 2 As the ozone layer protects us from harmful UV radiation, the international community chose to ban the use if CFCs. Unfortunately many CFCs are very unreactive and will persist in the atmosphere for hundreds of years. Naming Naming halogenoalkanes is relatively simple and follows the IUPAC convention… • Prefix with fluoro, chloro, ____________ or _____________ depending on the halogen • More than one halogen group can be indicated by adding di, tri or tetra • Numbers are used to indicate the position of the halogen group in the chain
  7. 7. _________________ __________________ __________________ __________________ Primary, secondary or tertiary? Just like alcohols, halogenoalkanes can be either primary, secondary or tertiary depending on the number of alkyl groups attached to the carbon with the halogen group… Primary halogenoalkanes Chloroethane Primary halogenoalkanes have the general structure RCH2X. There are two hydrogens and one alkyl group attached to the same carbon as the halogen group Secondary halogenoalkanes 2-chloropropane Secondary halogenoalkanes have the general structure RR1CHX. There is one hydrogen and two __________ groups attached to the same carbon as the halogen group Tertiary halogenoalkanes 2-bromo-2-methylpropane Tertiary halogenoalkanes… Primary, secondary and tertiary halogenoalkanes react very differently. Luckily there is a test, which
  8. 8. can be used to identify which is present using silver nitrate solution. We will cover the test in the reactions section under hydrolysis. Reactions - hydrolysis, elimination and substitution As mentioned on the previous page, the position of the halogen in the hydrocarbon chain affects reactivity, as does the type of halogen present… Bond Average bond energy (kJmol -1 ) C-X bonds become ______________ down the C-F 467 C-Cl 346 group, meaning that fluorocarbons are very C-Br 290 _______________ and iodocarbons are C-I 228 relatively ______________ a. Hydrolysis - ‘splitting with water’ Halogenoalkanes react with water or hydroxide (OH - ) ions in a reaction known as hydrolysis. The products are an alcohol and a hydrogen halide : RX + H2O → ROH + HX RX + OH- → ROH + X- e.g. CH3Cl + H2O → _____ + _____ CH3CH2CH2Br + H2O → _____________ + _____ NB Reactivity depends on the H-X bond strength as shown in the table above. Fluorocarbons are the least likely to undergo hydrolysis Test for primary, secondary or tertiary - silver nitrate Procedure • Add 1 cm3 of silver nitrate and 1 cm3 ethanol (__________) to each tube and warm • Add 5 drops of the different halogenoalkanes and observe Explanation • Halogenoalkanes react with water to form an _____________ and 1-bromobutane 2-bromobutane 2-bromo-2- some _____________ ions methylbutane • Silver nitrate (AgNO3) reacts with Primary the halide ions to form an insoluble _______________ • Rate of reaction: _________ > _________ > _________
  9. 9. b. Reaction with potassium hydroxide Potassium hydroxide (KOH) can react with halogenoalkanes in one of two ways depending on whether it is dissolved in water or ethanol : Conditions Type of reaction Product i) Reflux heating with aqueous Substitution Alcohol KOH (dissolved in _________) ii) Reflux heating with alcoholic KOH (dissolved in Elimination Alkene __________) For example: i) CH3CHBrCH3(l) + KOH(aq) → ____________ + ______ 2-bromopropane aqueous KOH ii) CH3CHBrCH3(l) + KOH(alc) → ____________ + ______ 2-bromopropane alcoholic KOH c. Reaction with alcoholic ammonia There is a ________ pair of electrons on the ____________ atom of ammonia (NH3), this means it can act as a nucleophile and replace the halogen atom in a halogenoalkane: i) CH3CH2I + NH3 → CH3CH2NH2 + HI ethylamine Ethylamine also has a lone pair of electrons on nitrogen so it can react further with iodoethane and there is a chain reaction*: ii) CH3CH2I + CH3CH2NH2 → (CH3CH2)2NH + HI iii) CH3CH2I + (CH3CH2)2NH → (CH3CH2)3N +HI iv) CH3CH2I + (CH3CH2)3N → (CH 3 CH 2 ) 4 N + I - tetraethylammonium iodide *Skeletal formulae for the amine products reactions i-iv respectively:
  10. 10. Spider diagram of A-level organic chemistry reactions Task - Produce your own spider diagrams to represent the reactions of alcohols and halogenoalkanes that we have covered. Be sure to include: • Reactants • Conditions • Products