CH221 CLASS 11
CHAPTER 6: ALKENES – STRUCTURE AND REACTIVITY
Synopsis. This class serves as an introduction to the chemistry of alkenes, which is dominated by the presence of C=C. Occurrence, industrial aspects, degree of unsaturation, nomenclature, electronic structure, cis-trans isomerism (including Z/E notation) and stability are considered here.
Introduction: Occurrence
An alkene is a hydrocarbon that contains a carbon-carbon double bond. Olefin is an alternative name (Greek: ‘oil-formers’). Like alkanes and cycloalkanes, alkenes are of widespread occurrence in nature: from ethylene itself to terpenes and carotenes. Other compounds containing C=C are also common in nature. Ethylene is produced by plants and acts as a hormone that stimulates the ripening of fruit.
Industrial Aspects
Ethylene and propylene, derived from the cracking of petroleum fractions, are the two most important organic chemicals to be produced industrially. They are used to synthesize a large number of industrially important compounds, as illustrated on the next page.
Ethylene and propylene are produced by the cracking of alkanes, as already described. Thermal cracking is a radical process, whereas catalytic cracking involves the formation of carbocations. In either case, the break up of a large molecule into many fragments at high temperatures results in a large value for TDSo, so that DGo for these reactions is negative (favorable), despite the positive (endothermic) values of DHo (remember: DGo = DHo - TDSo).
Degree of Unsaturation
An alkene has two fewer hydrogen atoms than the corresponding alkane, because of the presence of the double bond: the general formula for alkenes is CnH2n and they are said to be unsaturated. Another situation that leads to fewer hydrogens in a formula than the corresponding alkane is the presence of rings. It is possible to work back from the formula of an unknown and determine its degree of unsaturation (d.u.), alternatively called its double bond equivalents.
For example, C6H10 compares with alkane C6H14, the difference is 2 x H2 Þ 2 d.u. Possible identities include -
Similar deductions can be made for compounds containing heteroatoms (halogens, O, N, etc), according to the rules below:
Halogens (F, Cl, Br, I) - Add the number of halogens to the number of hydrogens in the formula
Oxygen – Ignore the number of oxygens in the fomula
Nitrogen – Subtract the number of nitrogens from the number of
hydrogens in the formula
Example
C2H4NOCl º C2H4 (cf. C2H6) Þ 1 d.u.
Possible identity: ClCH2CONH2 (chloroacetamide)
CMore will be said of this in the structure determination section of Organic Chemistry II.
Nomenclature
To name alkenes, carry out the following steps.
Further Examples
Cycloalkenes are named similarly, with the double bond taking the lowest number, in the absence of more senior groups, such as hydroxyl and carbonyl.
Common (trivial) names are still used for some alkenes (e.g. ethylene, propylene) and for common alkenyl groups:
CH2= (methylene) CH2=CH- (vinyl) CH2=CH-CH2- (allyl)
Electronic Structure of Alkenes and Cis-Trans Isomerism
This was discussed in class 2, but it is summarized here, with emphasis on the p bond, which dominates the chemistry of alkenes:
The energy barrier to rotation around C=C is very high (~ 268 kJ mol-1) compared with that for rotation around C-C (³12 kJ mol-1).
It is this very high energy barrier that allows the existence of the form of stereoisomerism that is associated with alkenes - cis-trans isomerism: certain alkenes can exist as cis and trans isomers:
The names cis and trans are the original names for these isomers, but this nomenclature becomes ambiguous for tri- and tetrasubstituted alkenes, as shown below.
The Newer (Z/E) Nomenclature
To avoid ambiguity like that above, a new system, the Z/E system, of naming was developed. This system, devised by Cahn, Ingold and Prelog, involves priority labeling of the substituents attached to the alkene carbon atoms, according to a strict set of rules based upon atomic number. Application of these rules (see textbook, p 178) leads to the following priority sequence for some common functional groups. A more extensive list is given overleaf.
I > Br > Cl > SO3R > SO3H> SO2R > SOR > SR > SH > F > OCOR > OR > OH > NR2 > NHCOR > NHR > NH2 > CO2R > CO2H > CONH2 > COR > CHO > CH2OH > Ph > CR3 > CHR2 > CH2R > CH3 > 3H > 2H > H > lone pair
The priority is based firstly upon the atomic number of the atom actually bonded to the alkene carbon atom. If there is still ambiguity (e.g. if two substituents are bonded via the same element, say carbon), the next atom is considered and so on, until the ambiguity vanishes.
To name an alkene according to the Z/E notation, carry out the following steps>
1. Assign priority (1,2 or a, b) to the substituents, separately, at either end of the double bond, according to the Cahn-Ingold-Prelog rules.
2. Name Z if the two top priority groups are on the same side of the double bond or E if they are on opposite sides (This comes from the German, Z from zusammen (together) and E from entegen (opposite)).
Priority Symbol Name(s)
1 / I / Iodo
2 / Br / Bromo
3 / SeCH3 / Methylselenanyl
4 / Cl / Chloro
5 / SO2CH3 / Methylsulfonyl
6 / SCH3 / Methylthio
7 / SH / Mercapto
8 / F / Fluoro
9 / OCOCH3 / Acetoxy
10 / OPh (OC6H5) / Phenoxy
11 / OCH2Ph / Benzyloxy
12 / OCH3 / Methoxy
13 / OH / Hydroxy
14 / NO2 / Nitro
15 / NO / Nitroso
16 / N(CH3)2 / Dimethylamino
17 / NHCOPh / Benzoylamino
18 / NHCOCH3 / Acetylamino
19 / NHPh / Phenylamino (anilino)
20 / NHCH3 / Methylamino
21 / NH3+ / Ammonio
22 / NH2 / Amino
23 / COOC(CH3)3 / tert-Butoxycarbonyl
24 / COOCH3 / Methoxycarbonyl
25 / COPh / Benzoyl
26 / COCH3 / Ethanoyl (acetyl)
27 / CHO / Methanoyl (formyl)
28 / 2-CH3C6H4 / o-Tolyl
29 / 4-NO2C6H4 / p-Nitrophenyl
30 / 4-CH3C6H4 / p-Tolyl
31 / Ph (C6H5) / Phenyl
32 / CºCH / Ethynyl
33 / (CH3)C=CH2 / 1-Methylethenyl (isopropenyl)
34 / C(CH3)3 / 1,1-Dimethylethyl (t-butyl)
35 / CH=CHCH3 / 1-Propenyl
36 / C6H11 / Cyclohexyl
37 / CH(CH3)CH2CH3 / 1-Methylpropyl (s-butyl)
38 / CH=CH2 / Ethenyl (vinyl)
39 / CH(CH3)2 / 1-Methylethyl (isopropyl)
40 / CH2Ph / Phenylmethyl (benzyl)
41 / CH2CºCH / 2-Propynyl
42 / CH2C(CH3)3 / 2,2-Dimethylpropyl (neopentyl)
43 / CH2CH=CH2 / 2-Propenyl (allyl)
44 / CH2CH(CH3)2 / 2-Methylpropyl (isobutyl)
45 / CH2CH2CH(CH3)2 / 3-Methylbutyl (isopentyl)
46 / CH2CH2CH2CH3 / Butyl
47 / CH2CH2CH3 / Propyl
48 / CH2CH3 / Ethyl
49 / CH3 / Methyl
50 / T (3H) / Tritium
51 / D (2H) / Deuterium
52 / H (1H) / Hydrogen (protium)
53 / : / Lone pair of electrons
Class Questions
1. Assign Z or E configurations to the following alkenes.
(a): Z (b): E (c): Z (d): E
3. Give full systematic names for the following: use Z/E notation
(Z)-2-butene (2Z, 4E)-2,4-heptadiene
Stability of Alkenes
Stability of Cis and Trans (Z and E) Isomers
Interconversion of cis and trans isomers of alkenes does not normally occur in the absence of some vigorous agent, because of the highly restricted rotation around the C=C bond. However, interconversion (isomerization) can be achieved by the use of a chemical reagent, such as a strong acid catalyst, or in some cases, by the action of heat or light. When this occurs, the trans isomer is nearly always favored over the cis isomer in the equilibrium mixture, as illustrated below for the isomerization of 2-butene.
Isomerizations like the one above, can be investigated quantitatively by chromatography, NMR and other methods, but thermodynamic data can also be used as further evidence. E.g. for 2-butene:
Cis / TransDHo(combustion) kJ mol-1 / -2685.5 / -2682.2
DHo (hydrogenation) kJ mol-1 / -120.0 / -116.0
All the evidence shows that, in the big majority of cases, trans alkenes are thermodynamically more favorable than cis alkenes. The major reason for this is thought to be the greater steric hindrance between cis substituents:
Stability of Substituted Alkenes
It was shown previously that measurement of enthalpies of hydrogenation of alkenes is a good criterion of the relative stability of cis and trans isomers. This criterion can also be used to compare the stabilities of alkenes with different extents of substitution (see textbook, p. 183, Table 6.2). All the data suggests that the general order of stability is as follows:
R-CH=CH2 R2C=CH2 (terminal) R2C=CHR R2C=CR2
Monosubstituted RCH=CHR trisubstituted tetrasubstituted
disubstituted
This order is best explained by supposing hyperconjugation stabilization is most extensive with the greater number of substituents.
E.g. In resonance terms,
CH3-CH=CH-CH3 « CH3-CH=CH-CH2. .H « CH3CH.-CH=CH2 .H etc
Or, in molecular orbital terms,
Also, (sp2-sp3)s bonds are stronger than (sp3-sp3)s bonds, hence the greater the number of (sp2-sp3)s bonds (the greater the number of substituents), the greater the stability.