1-bromo-1-chloro-2,2-dimethylpropene: Looking at the double bond, we can see that the right carbon has identical groups (methyls) attached to it, so there CANNOT be geometric (cis-trans) isomers.1,1-dichloroethene: The name itself gives this one away. There are 2 chlorines on the left carbon so there can’t be geometric isomers.1,2-dibromoethene: This molecule has cis-trans geometric isomers. It doesn’t matter that both carbons have the same two groups attached, as long as NEITHER carbon has 2 identical groups attached to it.3-ethyl-2-methyl-2-hexene: Because the double bond begins at carbon 2 and there is also a methyl attached to carbon 2, the left carbon has identical (methyl) groups and there cannot be cis-trans isomers.
Hydrogenation: A-B is H-H. Both A and B represent a hydrogen atom.Halogenation: A-B is X-X (F-F or Cl-Cl or Br-Br or I-I). Both A and B represent X (F or Cl or Br or I).Hydration: A-B is H-OH. So A represents H and B represents OH (or vice versa: A represents OH and B represents H).Hydrohalogenation: A-B is H-X (H-F or H-Cl or H-Br or H-I). So A represents H and B represents X (F or Cl or Br or I).
Linear molecules pack together very efficiently, which maximizes the intermolecular forces. Saturated oils have long, linear chains of carbons.Nonlinear molecules don’t pack together as efficiently and experience weaker intermolecular forces. The weaker the forces, the lower the melting point.Natural oils (vegetable oils) are unsaturated so the molecules are nonlinear. These oils are liquid at room temperature.Hydrogenation adds hydrogens to the double bonds, making the molecules more nearly linear. This increases the intermolecular forces and raises the melting point. So, the more an oil is hydrogenated, the higher its melting point and the more solid it is at room temperature.
Chapter 11The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatics
1. StructureAlkenes are hydrocarbons with a double bond. CnH2n
1. StructureAlkynes are hydrocarbons with a triple bond. CnH2n-2
1. StructureAlkenes and alkynes are unsaturated (don’t have themaximum number of hydrogens bonded to each carbon). Tegrity lecture video
1. Physical properties In each case, the alkyne has a higher boiling point than the alkene. Its structure is more linear. The molecules pack together more efficiently. Intermolecular forces are stronger. Tegrity lecture video
2. NomenclatureThe root name is based on the longest chain thatincludes both carbons of the multiple bond.The –ane ending is changed to –ene for double bondsand –yne for triple bonds. ethyne ethene propyne propene
2. NomenclatureThe chain is numbered from the end nearest the multiplebond. 2-pentyne 1-butene [not 3-pentyne] [not 3-butene]The position of the multiple bond is indicated with thelower-numbered carbon in the bond.
2. NomenclatureDetermine the name and number of each substituentand add in front of the name of the parent compound. 5-chloro-4-methyl-2-hexene 2,6-dimethyl-3-octene 5-bromo-4-ethyl-2-heptene
2. NomenclatureAlkenes with more than one double bond are called alkadienes (2 double bonds) alkatrienes (3 double bonds) etc…Each double bond is designated by its lower-numberedcarbon. 2,4-hexadiene
2. NomenclatureCycloalkenes must be numbered so the double bond isbetween carbons one and two. 3-chloro-cyclopentene 4-ethyl-5-methylcyclooctene
2. NomenclatureName the following compounds. CH3CH=C(CH2CH3)2 H2C=C-CH2-CH=CH2 pencast
2. NomenclatureName the following compounds. pencast
2. NomenclatureWrite a structural formula for each of the followingcompounds. 1-hexene 1,3-dicholoro-2-butene 4-methyl-2-hexyne pencast 1,4-cyclohexadiene
2. NomenclatureDraw a structural formula for each of the followingcompounds: 1-bromo-3-hexyne 2-butyne pencast dichloroethyne 9-iodo-1-nonyne
3. Geometric isomers Rotation around a double bond is restricted, in much the same was as rotation is restricted for the cycloalkanes. In the alkenes, geometric isomers occur when there are two different groups on each of the double-bonded carbon atoms. 1,2-dichloroethene
3. Cis-trans isomers If both constituents are on the same side of the double bond, the isomer is cis-. cis-1,2-dichloroethene If the constituents are on opposite sides of the double bond, the isomer is trans-. trans-1,2-dichloroethene
3. Cis-trans isomers Alkenes without substituents also may exhibit cis-trans isomerism. cis-4-octene trans-4-octene
3. Cis-trans isomers In order for cis and trans isomers to exist, neither double- bonded carbon may have two identical substituents. 2-methyl-2-butene no cis/trans isomerism 1-butene no cis/trans isomerism
3. Cis-trans isomers Which of the following compounds can exist as geometric isomers? 1-bromo-1-chloro-2,2-dimethylpropene 1,1-dichloroethene 1,2-dibromoethene 3-ethyl-2-methyl-2-hexene
5. Reactions of alkenes and alkynes The most common reactions of alkenes and alkynes are addition reactions. Hydrogenation: addition of H2 Halogenation: addition of X2 Hydration: addition of H2O Hydrohalogenation: addition of HX
5. General addition reaction A double bond consists of a sigma bond: two electrons concentrated on a line between the two connected atoms; a pi bond: two electrons concentrated in planes above and below the sigma bond.
5. General addition reaction In an addition reaction, the pi bond is lost and its electrons become part of the single bonds to A and B.
5. General addition reaction For hydrogenation, halogenation, hydration, and hydrohalogenation, identify the A and B portions of what is being added to the double bond. hydrogenation, H2 halogenation, X2 (where X = F, Cl, Br, or I) hydration, H2O hydrohalogenation, HX (where X = F, Cl, Br, or I)
5. Hydrogenation In hydrogenation of an alkene, one molecule of hydrogen (H2) adds to one mole of double bonds. Reaction conditions: platinum, palladium, or nickel catalyst [sometimes] heat and/or pressure
5. Hydrogenation In hydrogenation of an alkyne, two molecules of hydrogen (H2) add to one mole of triple bonds. Reaction conditions: same as for alkenes.
5. Hydrogenation Compare the products resulting from the hydrogenation of trans-2-pentene and cis-2-pentene. pencast
5. Hydrogenation Compare the products resulting from the hydrogenation of 1-butene and cis-2-butene. pencast
5. Vegetable oil and margarine Why does hydrogenation make oils more solid? MP = 13-14oC MP = 69.6oC MP = 62.9oC
5. Halogenation In halogenation of an alkene, one mole of a halogen (Cl2, Br2, I2) adds to one mole of double bonds. Since halogens are more reactive than hydrogen, no catalyst is needed.
5. Halogenation In halogenation of an alkyne, two moles of a halogen (Cl2, Br2, I2) add to one mole of double bonds.
5. Halogenation Draw the structure and write a balanced equation for the halogenation of each of the following compounds. 3-methyl-1,4-hexadiene 4-bromo-1,3-pentadiene 3-chloro-2,4-hexadiene pencast
5. Halogenation A solution of bromine in water has a reddish-orange color. A simple test for the presence of an alkene or alkane is to add bromine water. If a double or triple bond is present, the bromine will be used up in a halogenation Test of cyclohexane reaction and the color will and cyclohexene disappear.
5. Hydration In hydration, one mole of water (H2O) is added to one mole of double bonds. A trace of acid is required as a catalyst.
5. Hydration Unlike hydrogenation and halogenation, hydration is not a symmetric addition to a double bond. If the double bond is not symmetrically located in the molecule, there are two possible hydration products.
5. Hydration The predominant product is determined by Markovnikov’s rule: The rich get richer. OR: The carbon that already has more hydrogens will get the hydrogen from the water. Hydration of propene: + H 2O
5. Hydration Write a balanced equation for the hydration of each of the following compounds: 2-butene 2-ethyl-3-hexene pencast 2,3-dimethylcyclohexene Alkynes undergo a much more complicated hydration that you don’t need to remember at this time!
5. Hydrohalogenation Like hydration, hydrohalogenation is an asymmetric addition to a double bond. Hydrohalogenation also follows Markovnikov’s rule.
6. Aromatic compounds Consider the following molecular formulas for unsaturated hydrocarbons: Hexane (all single bonds): C6H14 Cyclohexane (one ring): C6H12 Hexene (one double bond): C6H12 Hexadiene (two double bonds): C6H10 Cyclohexene (one ring, one double bond): C6H10 Hexatriene (three double bonds): C6H8 Cyclohexadiene (one ring, two double bonds): C6H8
6. Aromatic compounds The molecular formula for benzene is C6H6. The structure must be highly unsaturated. One ring, three double bonds? Reactions of benzene: Benzene does not decolorize bromine solutions. Benzene does not undergo typical addition reactions. Benzene reacts mainly by substitution. The first three items are opposite from what is expected from unsaturated compounds. The last item is identical to what is expected for alkanes.
6. Benzene structure The benzene ring consists of: six carbon atoms joined in a planar hexagonal arrangement with each carbon bonded to one hydrogen atom. Two equivalent structures proposed by Kekulé are recognized today as resonance structures. The real benzene molecule is a hybrid with each resonance structure contributing equally to the true structure.
6. Benzene structure Sigma and pi bonding in benzene: The sharing of six electrons over the entire ring gives the benzene structure extra stability. Removing any one of the six electrons would destroy that stability.
6. Nomenclature Most single-substituent compounds are named as derivatives of benzene. Bromobenzene Ethylbenzene
6. Nomenclature A few “common” names have been adopted as IUPAC nomenclature. toluene phenol aniline xylene (any benzene ring with two methyl groups)
6. Nomenclature There are three ways for the methyl groups on xylene to be arranged. 1,2 [ortho-xylene] 1,3 [meta-xylene] 1,4 [para-xylene]
6. Nomenclature The substituent created by removing one hydrogen from the benzene ring is called phenyl-. 2-phenylhexane 3-phenylcyclopentene
6. Nomenclature The substituent consisting of a –CH2 attached to a benzene ring is called benzyl-. Benzyl chloride
6. Polynuclear aromatic hydrocarbons These consist of rings joined along one side. Good news! You don’t have to memorize these names!
6. Reactions of benzene Because of the stability of benzene’s ring structure, only substitution reactions are characteristic. Halogenation: substitution of one or more halogen atoms for hydrogen atoms. Cl2 requires FeCl3 catalyst. Br2 requires FeBr3 catalyst. Nitration: substitution of one or more nitro- (-NO2) groups for hydrogen atoms. Requires nitric acid and concentration sulfuric acid. Sulfonation: substitution of one sulfonic acid (-SO3H) group for a hydrogen atom. SO3 reactant and concentration sulfuric acid.
7. Heterocyclic aromatic compounds Heterocyclic aromatic compounds have at least one non- carbon atom incorporated in an aromatic ring or polynuclear aromatic compound. Many of these compounds are biologically important. Components of DNA and RNA Components of hemoglobin and chlorophyll Pharmaceuticals pyridine