Chemistry 110
Bettelheim, Brown, Campbell & Farrell
Ninth Edition
Introduction to General,
Organic and Biochemistry
Chapter 12
Alkenes & Alkynes
Chapter 12
Alkenes are hydrocarbons which have one or
more carbon-carbon double bonds.
VSEPR
angles 120E
VSEPR
angles 180E
Alkynes are hydrocarbons which have one or
more carbon-carbon triple bonds. These have
little import to biochemistry and will not be
studied further in this course.
Fig. 12.UN, p.314
Physical Properties of Alkenes and Alkynes
¾Alkenes and alkynes are nonpolar compounds.
¾The only attractive forces between their molecules
are London dispersion forces.
¾Their physical properties are similar to those of
alkanes with the same carbon skeletons.
¾Alkenes and alkynes are insoluble in water but
soluble in one another and in nonpolar organic
liquids.
¾Alkenes and alkynes that are liquid or solid at room
temperature have densities less than 1 g/mL; they
float on water.
1
Geometric (cis-trans) Isomerism in Butenes
No geometrical
isomers possible.
H
CH3CH2
H
H3C
C C
C C
H
H
1-Butene
CH3
Different
Connectivity
CH3
C
H
CH3
C
H
Same connectivity but
different geometry.
C
H
H
H3C
Methylpropene
H
C
CH3
cis-2-butene
trans-2-butene
geometric isomers
Geometric (cis-trans) Isomerism
The groups can be different!
1
5
4
CH3
2
3
C
CH2CH3
1
CH3
H
2
3
H
C C
C
CH2CH3
H
H
4
5
cis-2-pentene
trans-2-pentene
geometric isomers
Cl
CH2CH3
C
H
Cl
C C
C
H
H
cis-1-chloro-1-butene
H
CH2CH3
trans-1-chloro-1-butene
IUPAC Systematic Naming of Alkenes
•Longest continuous carbon chain containing the double bond = Parent
Alkene (PA; even if a longer chain without the double bond exists!); assign
numbers counting from the end giving the first carbon of the double bond
the lowest number.
•Locate any chain branching off PA, name as alkyl group.
•For multiple branching – Locate identical alkyl groups, indicate their
number using bi, di (2); tri (3); tetra (4) … Locate different alkyl groups.
•Construct the name by locating the double bond with a number preceeding
the PA separated by hyphens, place the alkyl groups in alphabetical order
(ignoring Greek prefixes, dimethyl = m) preceded by locator numbers
separated by hyphens from words and commas from other numbers, a
locator number for each alkyl group, i. e. 2,2-dimethyl.
1
2
CH3 CCH3
4
5
6
7
CH3CHCCH2CH2CHCH3
3
CH3
3-isopropyl-2,6dimethyl-2-heptene
CH3
2
IUPAC Systematic Naming of cycloAlkenes
•Largest carbon ring containing the double bond = Parent cycloalkene
(PCA) – use cyclo+(name of straight chain having same number of carbon
atoms as the ring). Count the carbon atoms of the double bond 1 and 2 in the
direction giving the nearest substituent the lowest number.
•Locate any chain branching off PCA, name as alkyl group.
•For multiple branching – Locate identical alkyl groups and indicate their
number using bi, di (2); tri (3); tetra (4) … Locate different alkyl groups.
•Construct the name by placing the alkyl groups in alphabetical order
(ignoring Greek prefixes, dimethyl = m) preceded by locator number
separated by hyphens from words and commas from numbers. There
should be a locator number for each alkyl group.
CH(CH3)2
3
2
4
1
5
6
5-ethyl-3-isopropylcyclohexene
CH2CH3
Addition Reactions to the Double Bond
Addition of Cl2 and Br2
Addition takes place readily at room temperature
¾reaction is generally carried out using pure reagents, or
mixing them in a nonreactive organic solvent
CH3 CH= CHCH 3
+
Br 2
2-Butene
+ Br2
CH 2 Cl2
Br Br
CH 3 CH- CHCH3
2,3-Dibromobutane
Br
CH2 Cl2
Cyclohexene
Br
1,2-Dibromocyclohexane
¾addition of Br2 is a useful qualitative test for the presence of
a carbon-carbon double bond
¾Br2 has a deep red color; dibromoalkanes are colorless
Addition Reactions to the Double Bond
H
H
H
C C
H + HCl
H
H H
cis-2-butene
H
C
C
H
H Cl
chloroethane
ethene,
ethylene
CH3 C C CH3 + HOH
H
H H
H2SO4
CH3 C C CH3
H OH
2-butanol
3
Addition Reactions to the Double Bond
H
H
O O
H C C H + S
HO OH
ethene
H
H
H
C C
H
O OH
ethyl hydrogen sulfate S
O O
H
H
CH3CH2 C
H
CH2 + HOH
CH3CH2 C CH2
HO H
2-butanol
1-butene
Unsymmetrical Addition Reactions
Markovnikov’s Rule – The carbon having the most
hydrogen atoms gets the hydrogen atom.
CH3CH2CH CH2 + HCl
CH3CH2CH CH2
1-Butene
Cl H
2-chlorobutane
CH3
CH3
CH3C
CH3C CH2 + HOH
Methylpropene
CH2
HO H
2-methyl-2-propanol
Unsymmetrical Addition Reactions
Non-Markovnikov
H3C CH2CH3
H C C H
H3C CH2CH3
H C C H + HBr
cis-2-pentene
H Br
3-bromopentane 50%
H3C CH2CH3
H C C H
Br H
2-bromopentane 50%
4
How Addition Reactions Occur
Positively Polarized Hydrogen Atoms in molecules
react with alkenes to form Carbocations
H G + CH3CH CH2
CH3CH CH3 + G
Carbocation Stability:
tertiary > secondary > primary > methyl
CH3CH CH3
CH3CH2 CH2
secondary >> primary cation
Carbocations – Lewis Structure
H
H
HC C CH
H H H
The center carbon has only 3
bonds and an incomplete octet
of electrons.
G
H GH
HC C CH
HH H
Such molecules are strong
Lewis Acids and are readily
attacked by molecules having
lone pairs of electrons – Lewis
Bases
Carbocations – Lewis Structure
H
H
The water molecule (and many
H C C C H others) is too weak a Bronsted
acid to protonate an alkene and
H H H
make a carbocation.
O H A strong acid catalyst is added to
H
form the carbocation.
H
The water molecule then donates
H OH
its lone pair (and the OH bearing
H C C C H it) to the carbocation forming an
HH H
oxonium ion. It then loses a
+ H
proton to form the alcohol.
5
Carbocation Stability Directs Addition
To Double Bonds
CH3
CH3
H
H3C C CH2
H3C C CH3
CH3CH2CH CH2 H
CH2
CH3CH2CH CH3
H
CH2
CH2
CH3
Polymers Can Be Made By Carbocation
Addition to Alkenes
CH3
HC CH2
CH3
CH3
H
HC CH3
CH3
CH3
HC CH2 + HC CH3
CH3
CH3
CH3
HC CH2CHCH3
CH3
HC CH2 + HC CH2CHCH3
CH3
CH3
HC CH2(CHCH2)2H
repeating unit
6