Carbocations are also stabilised by resonance when the electron-poor
cationic carbon is adjacent to an electron-rich -system:
H
H
+
C
C
H
H
+ H
C
H + C
H
C
C
H
H
H
Allyl carbocation
H + H
C
H
C
H
H
C
H
H
C
H
+
+
+
Benzyl carbocation
Not surprisingly, the order of reactivity of the substrates in SN1
reactions is exactly the same as the overall stability order for
carbocations:
3° > 2° ≈ benzyl ≈ allyl > 1° > Methyl
(2) The effect of the Nucleophile:
The rates of SN1 reactions are independent of the nature of the
nucleophile because the nucleophile is not involved in the ratedetermining step.
Partial comparison of SN1 and SN2 substitution reactions:
Variable
Substrate structure:
Methyl or 1°
SN1
SN2
Does not occur except Common
for 1° allyl and 1°
benzyl
Secondary
Mainly allylic and Sometimes
benzylic systems
Tertiary
Stereochemistry
Nucleophile
Common
Racemisation
Rate does not depend
on the concentration
or the nature of the
nucleophile.
Does not occur
Inversion
Rate does depend on
the concentration and
the nature of the
nucleophile - SN2 is
especially favoured
by anionic nucleophiles.
Questions for home study and discussion at the Class Tutorial:
(1) How does the nature of the leaving-group affect SN2 reactions?
(2) How does the nature of the solvent affect SN2 reactions?
(3) How does the nature of the leaving-group affect SN1 reactions?
(4) How does the nature of the solvent affect SN1 reactions?
Alkenes (also known as olefins) - compounds with a carbon-carbon
double bond.
The simplest alkene - ethene or ethylene: H2C=CH2
CH3CH2 OH
Ethanol
H2C CHCl
Vinyl chloride
CH3CHO
Acetaldehyde
H2C
CH2
2.6 x 10 7 tons per year in US
HOCH2CH2 OH
Ethylene glycol
CH3COOH
Acetic acid
Electronic structure of the carbon-carbon double bond:
H
H
C
H
H
H
H
C
Barrier ca. 3 kcal mol-1
H
C
H
H
H
Staggered
H
CH3
C
C
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
Eclipsed
H
H
H
H
H
H
C
C
C
CH3
Staggered
H
CH3
H
Eclipsed
Different conformations of the same compound.
Interconvert very rapidly at room temperature.
Cannot be separated or isolated.
H
H
C
H
C
H
C
Rotate by 90°
around -bond
H
Barrier 64 kcal mol-1
H
H
C
H
CH3
H
H
C
C
C
C
H
CH3
trans(Latin: across)
H
CH3
CH3
cis(Latin: on the same side)
The cis- and trans- isomers of alkenes cannot interconvert due to the
high barrier to rotation about the double bond - they are different
compounds.
Nomenclature - the systematic rules for naming alkenes:
(1) Find the longest chain of carbon atoms that includes the double
bond - then name as for the corresponding saturated hydrocarbon
(alkane) but use the termination -ene rather than -ane:
CH3
H2C
CH
CH2CH3
H2C
propene
H3C
butene
CH3
HC
CH
CH3
CH3
C
CH
C
CH3 CH2
butene
CH2CH3
hexene
(2) Number the carbon atoms of the longest chain, beginning at the end
nearest to the double bond:
3
CH3
2
1
3
CH
H2 C
1-butene
4
1
C
CH
3
2
H
H
CH3
H3C
HC
CH
H2C
1-propene
CH2CH3
2
1
4
CH3
1
2-butene
2
C
3
2-pentene
CH2CH3
4
5
(3) Indicate the position of any groups (substituents) attached to the
parent alkene chain by the appropriate number:
CH3
CH3
C
CH3 CH2
1
3
1
C
3
H2C
4
CH2CH3
2
5
6
2
CH3
C
CH3
2-methyl-1-propene
3,4-dimethyl-3-hexene
3
3
1
H2C
2
CH2Cl
1
ClHC
C
CH3
3-chloro-2-methyl-1-propene
2
CH2Cl
C
CH3
1,3-dichloro-2-methyl-1-propene
Some non-systematic names still in common use:
H2C
CH2
H2C
ethylene
H2C
methylene
H2C
CH
vinyl-
CH CH2
allyl-
CH2
H2C
CH Cl
H2C
vinyl chloride
CH CH2
OH
allyl alcohol
methylenecyclohexane
Reading exercise: Read McMurry (5th Ed.) pp. 192-194 for some
further aspects of the rules of alkene nomenclature and work through
the problems on pp. 194-195 .
The stability of alkenes:
Two important generalisations:
(1) Trans- alkenes are more stable than cis- alkenes.
(2) Alkene stability increases with increasing number of substituents on
the alkene carbons.
(1) Trans- alkenes are more stable than cis- alkenes.
Hoisomerisation = 1 kcal/mol
H
H
C
H
C
CH3
C
CH3
CH3
C
CH3
cis-2-butene
H
trans-2-butene
Hohydrogenation (kcals/mol) =
H2/Pd - 28.6
- 27.6 H2/Pd
CH3CH2 CH2CH3
Butane
Why is the trans- isomer more stable than the cis- ?
H
H
H
C
H C
H H
H
C
C
H H
H
H C
H H
C
C
Reduced steric crowding in the trans- isomer
H
C H
H
(2) Alkene stability increases with increasing number of substituents on
the alkene carbons.
CH3 CH2CH
CH2
CH3CH
CHCH3
1-butene
2-butene
Monosubstituted
Disubstituted
Hohydrogenation (kcals/mol) =
cis- - 28.6
trans- -27.6 H2/Pd
H2/Pd - 30.1
CH3CH2 CH2CH3
Butane
Why does increased substitution increase the stability of alkenes?
Two factors:
(A) Bond strength effects.
Substituted alkenes may contain three different kinds of C-C bonds:
sp2 - sp2 :
C
C
sp2 - sp3 :
C
C
C
C
sp3 - sp3 :
C
C
C
The order of stability of these bonds is sp2 - sp2 > sp3 - sp2 > sp3 - sp3.
It is the sp2 - sp3 > sp3 - sp3 difference that is important here.
CH3 CH2 CH CH2
CH3 CH CH CH3
1-butene
2-butene
1 sp 3-sp2 bond
1 sp3-sp3 bond
2 sp 3-sp2 bonds
0 sp 3-sp3 bond
More stable
The more alkyl substituents there are on the alkene carbons the more
stable sp3 - sp2 bonds are formed and the more stable is the alkene.
(B) Hyperconjugation
H
C
C
Filled C-H 
bonding orbital
C
Empty * C-C
antibonding orbital
Donating electrons
to:
The alkene is stabilised by a sideways-on (i.e. ) overlap of a filled C-H
-bonding orbital of an alkyl substituent with the empty * orbital of
the carbon-carbon double bond. The more alkyl substituents there are
on the alkene carbons the more stabilising interactions of this type can
occur and the more the alkene is stabilised. Hyperconjugation results in
a net lowering of energy, i.e. the alkene is stabilised.