The Structure and Synthesis of Alkenes

1. The Structure and Synthesis of Alkenes

2. Alkenes: • Are hydrocarbons with carbon-carbon double bonds. • Are also known as olefins. • C=C is considered to be a functional group because it is relatively reactive. • Double bonds (1.33 Å) are shorter than single bonds (1.54 Å).

3. Elements of Unsaturation • Alkenes are unsaturated because they are capable of adding hydrogen in the presence of a catalyst. Alkanes are saturated because they cannot react with anymore hydrogen. • Each pi bond of an alkene (or alkyne) or the ring of a cyclic compound decreases the number of hydrogen atoms in a molecular formula. This feature is called “element or degree or un-saturation”. • Each element of unsaturation corresponds to two fewer hydrogen atoms than the “saturated” formula.

4. Examples: propane saturated propene one element of unsaturation cyclopropane one element of unsaturation

5. A molecule represented by the formula C4H8 has one element of unsaturation since it is missing two hydrogens when compared to the saturated alkane formula (C4H10). There are five constitutional isomers of formula C4H8. 1-butene 2-butene isobuytlene cyclobutane methylcyclopropane

6. Elements of Unsaturation with Heteroatoms • Heteroatoms are atoms different from carbon and hydrogen. • Halogens: Halogen atoms are counted as hydrogen atoms. E.g. C2H4F2 has no elements of unsaturation. • Oxygen atoms: should be ignored when calculating elements of unsaturation. E.g. C2H4O has one element of unsaturation. • Nitrogen atoms: should be counted as half of a carbon atom. E.g. C4H9N should be treated as C4.5H(9+2).Thus C4H9N is two hydrogens short of the “saturated” formula, and it has one element of unsaturation.

7. Nomenclature of Alkenes • Similar to that of alkanes, except the ending of the root name corresponding to the longest continuous chain containing the double bond is changed from “ane” to “ene”. E.g. ethane becomes ethene, propane becomes propene, cyclohexane becomes cyclohexene. • When a chain contains more than 3 carbons, a number is used give the position of the double bond. The chain is numbered starting from the end closest to the double bond. E.g.: 2-pentene

8. Compounds with two double bonds are dienes, those with three double bonds are trienes, and those with four double bonds are tetraenes. Numbers are used to specifiy the locations of double bonds. 1,3-butadiene 1,3,5,7-cyclooctatetraene

9. With regard to numbering the chain, the double bond is given precedence over alkyl groups or halogens. 3-propyl-1-heptene 7-bromo-1,3,5-cycloheptatriene 3-methyl-1-butene

10. Alkenes named as substituents are called “alkenyl” groups. Some alkenyl substituents have common names. vinyl group methylene allyl group 3-methylene- cylcohexene 3-vinyl-1,5 hexadiene allyl chloride

11. ethene ethylene propene propylene • Some simple alkenes have common names. 2-ethylpropene isobutylene ethenylbenzene styrene

12. Cis/Trans Nomenclature • When two similar groups are on the same side of the double bond, the alkene is cis. When two similar groups are on opposite sides of the double bond, the alkene is trans. Note that if either carbon of the double bond holds two identical groups, the molecule cannot have cis and trans forms. cis-2-pentene trans-2-pentene 2-methyl-2-pentene (not cis or trans)

13. E/Z Nomenclature • When the cis/trans system fails to give an un-ambiguous name, the E/Z system is used. Consider each carbon of the double bond separately, and assign Cahn-Ingold-Prelog priorities to the two groups attached to each carbon. If the two groups of high priority are on the same side, the compound is Z; if they are on opposite sides, it is E. (Z)-1-bromo-1-chloro- propene (E)-1-bromo-1-chloro- propene

14. Commercial Uses of Alkanes • Used mostly in the production of polymers. • Serve as intermediates in the synthesis of drugs, pesticides and other valuable chemicals including ethanol, acetic acid, ethylene glycol and vinyl chloride. • Ethylene is uses as a plant hormone, accelerating the ripening of fruit.

15. Alkene Stability • Saytzeff’s Rule: the more highly substituted an alkene is, the more stable it is. In other words, alkyl groups attached to the double bonded carbons stabilize the alkene. • The two probable factors responsible for the stabilizing effect of alkyl groups to the double bond are the contribution of electron density to the pi bond by the alkyl groups, and the increased separation of bulky groups from one another (120o bond angle) in alkenes. • In general, trans isomers are more stable than cis isomers because of decreased steric interactions.

16. Cycloalkene Stability • The presence of a ring only makes a difference in stability if there is ring strain. Rings that are 5 carbons or larger easily accommodate double bonds, and such systems behave like straight chain alkenes. • The ring strain in cyclopropene and cyclobutene is greater than that in cyclopropane and cyclobutane. Thus cyclopropene and cyclobutene are less stable than the analogous alkanes. • In cyclic alkenes, the cis isomer is generally more stable than the trans. The trans isomer is rarely observed unless the ring contains 10 or more carbons.

17. Bredt’s Rule • Trans cycloalkenes are not stable unless there are at least 8 carbon atoms in the ring. Bredt’s rule states that a bridged bicyclic compound cannot have a double bond at the bridgehead position unless one of the rings contains at least 8 carbon atoms. Stable: although double bond is at bridgehead, system in not bridged. Unstable: double bond is at bridgehead and ring contains only 6 carbons.

18. Polarity • Alkenes are relatively nonpolar. They are insoluble in water, but soluble in nonpolar solvents such as hexane. Alkenes are slightly more polar than alkanes for two reasons: the pi bond electrons are more polarizable, thus contributing to instantaneous dipole moments, and the vinylic bond tends to be slightly polar, contributing to the permanent dipole moment. • In a symmetrical trans disubstituted alkene, the sum of the dipole moments is zero. In the analogous cis alkene, the vector sum of the two dipoles is directed perpendicular to the double bond. This results in a non-zero molecular dipole. The permanent dipole results in an increased bp.

20. Synthesis of Alkenes by Elimination Reactions • E2 dehydrohalogenation gives excellent yields with bulky secondary and tertiary alkyl halides that are poor SN2 substrates. A strong bulky base forces second order elimination by abstracting a proton. The bulkiness of the base hinders second order substitution. Tertiary halides are the best E2 substrates because they are prone to elimination and cannot undergo SN2 substitution.

21. Use of a Bulky Base • A bulky base can minimize substitution reactions by hindering the approach to attack at carbon. Yet, they can easily abstract a proton giving the elimination product. Some commonly used strong bulky bases are t-butoxide, triethylamine, diisopropyl amine, and 2,6 dimethylpyridine.

22. Examples of Strong Bulky Bases triethylamine diisopropylamine t-butoxide 2,6-dimethylpyridine

23. Example: Bromocyclohexene, a 2o halide, can undergo both substitution and elimination. E2 is favored over SN2 by using diisopropylamine as the base. Diisopropyl amine is too bulky to be a good nucleophile, but it acts as a strong base to abstract a proton.

24. Formation of the Hofmann Product • When non-bulky bases are used in eliminations in which there is a choice of proton to abstract, abstraction takes place to yield the most substituted alkene (Saytzeff product). However, steric hindrance may prevent a bulky base from abstracting the proton that would lead to the most stable alkene. In such cases, the less substituted alkene is formed (Hofmann product).

25. Example: Hofmann product Saytzeff product 29% 71% 28% 72%

26. Dehydrohalogenation by the E1 Mechanism • E1 dehydrohalogenation usually takes place in a good ionizing solvent such as an alcohol or water, without a strong nucleophile or base to force 2nd order kinetics. The substrate is usually as 2o or 3o alkyl halide. This reaction is generally accompanied by SN1 substitution because the nucleophilic solvent can also attack the carbocation directly.

27. Example:

28. Dehalogenation of Vicinal Dibromides • Vicinal dibromides can be converted to alkenes by reduction with either iodide ion or zinc in acetic acid. The reaction proceeds via an E2 mechanism, taking place through an anti-coplanar transition state.

29. Dehydration of Alcohols • Dehydration refers to the “removal” of water. An alcohol is protonated by an acid, and then water leaves yielding a carbocation that loses a proton to give an alkene.

30. Summary of Synthesis of Alkenes • Dehydrohalogenation by E2 mechanism: Saytzeff and Hofmann products are possible depending on the base used. • Dehydrohalogenation by E1 mechanism: both substitution and elimination products are formed. • Dehalogenation of vicinal dibromides: occurs with the treatment of vic-dibromide with I- or Zn in acetic acid via an E2 mechanism. • Dehydration: involves conversion of the poor -OH leaving group into the good leaving group H2O.