Chemistry 2213a Fall 2012 Western University
Topic 2b. Organic Structure #2: Alkenes & Alkynes
• Alkenes with one C=C bond have the formula C H , nh2ne alkynes with one
C≡C bond have the formula C H . Such compounds are said to be
unsaturated because they have less than the maximum number of hydrogens.
• Each reduction of two H from the general alkane formula represents one unit of
unsaturation (an index of hydrogen deficiency of one).
C5H 10 C5 8
saturate 1 unit of unsaturation 2 unis of unatuation
• So, each unit of unsaturation can be a π bond or a ring. Organic Structure #2: Alkenes & Alkynes 2
• When there are two or more multiple bonds, they can arranged as follows:
1.Isolated: two or more σ bonds between π bonds
2.Conjugated: multiple bonds alternate with a σ bond
3.Cumulated: two double bonds on a single carbon.
iolated conjugated cum ulated
What can be said about the 2p orbitals of a conjugated system?
• In systematic nomenclature, the
multiple bond (for C=C and C≡C)
is given the lowest possible
number on the parent chain. This
fixes the chain numbering.
C=C (alkene) has the suffix ene,
and C≡C (alkyne) the suffix yne. 3,5-dimethyl-2-hexene 3-n-butyl-1,4-pentadiyne Organic Structure #2: Alkenes & Alkynes 3
• Structurally, an alkene consists of two adjacent sp -hybridized atoms (trigonal
planar). The three sprbitals are used to form σ bonds. The remaining p
orbital overlaps sideways with the p from the adjacent atom to form one π
bond. π bonds have electron density above and below the molecular plane.
• An alkyne consists of two adjacent sp-hybridized atoms (linear).
alkenes, the hybrid orbitals are for σ bonds. The two π bonds originate from the
sideways overlap of two p orbitals from each of the atoms.
H C C H Organic Structure #2: Alkenes & Alkynes 4
B. cis-trans Isomerism of Alkenes
• A multiple bond prevents free rotation about the involved atoms. Such rotation
would require breaking the π bond and is not energetically favourable.As a
result, cis-trans (geometric) isomers can exist for non-symmetric alkenes.
H H H CH CH
C C C C
H3C CH 2H 3 H3C H
mp = –179°, bp = 38° mp = –136°, bp = 36°
• In this context, the terms cis-trans refer to the relative orientation of designated
substituents on the C=C bond.
• cis-trans also used to describe the orientations of substituents bonded to a
cycloalkane ring. Do not confuse the two different uses. Organic Structure #2: Alkenes & Alkynes 5
• Recall that if any one of the two carbons has two identical groups, there cannot
be cis-trans isomerism. The molecule shown below has no stereoisomers.
H C H H 3 Cl
C C same as C C
H C C l H C H
• Also, in ring compounds of 7 carbons or less, the double bond must be cis(as
defined by the carbon atoms).
• For every C=C in a molecule, there is a possibility of 2 cis-trans isomers. So,
for n C=C’s in a molecule, there is a maximum of 2ssible isomers.
• The actual number of isomers is decreased if one or more of the double bonds
is symmetrical (i.e. has two identical substituents on one of the C=C atoms).
o CH CH=CHCH=CHCH=CH has three C=C, but the o