1. Alkenes: Structure and Nomenclature
Unit 7

2. Unsaturated Hydrocarbons
Unsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds
Alkene: contains a carbon-carbon double bond and has the general formula CnH2n
Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2

3. Alkene and Alkyne Formulas
Alkenes contain one or more double bonds. The general molecular formula for an alkene with one double bond is CnH2n.
The fifth member of the alkene family, pentene, has 5 carbon atoms and 10 (2 × 5 = 10) hydrogen atoms, C5H10.
Alkynes contain one or more triple bonds. The general molecular formula for an alkyne with one double bond is CnH2n-2.
The fifth member of the alkyne family, pentyne, has 5 carbon atoms and 8 (2 × 5 – 2 = 8) hydrogen atoms, C5H8.

4. Unsaturated Hydrocarbons
Arenes: benzene and its derivatives
Basically unreactive

5. Structure of Alkenes A double bond consists of:
one sigma bond formed by the overlap of sp2 hybrid orbitals and one pi bond formed by the overlap of parallel 2p orbitals
the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°

6. Cis,Trans Isomerism in Alkenes
Cis,trans isomers: isomers that have the same connectivity but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond

7. Index of Hydrogen Deficiency
Index of hydrogen deficiency (IHD): the sum of the number of rings and pi bonds in a molecule
To determine IHD, compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds
the molecular formula of the reference hydrocarbon is CnH2n+2

8. Index of Hydrogen Deficiency
reference
- H
molecule )
IDH = 2
for each atom of a Group 7 element (F, Cl, Br, I), add one H
no correction is necessary for the addition of atoms of Group 6 elements (O,S) to the reference hydrocarbon
for each atom of a Group 5 element (N, P), subtract one hydrogen

9. Index of Hydrogen Deficiency
Problem: isopentyl acetate has a molecular formula of C7H14O2. Calculate its IHD
reference hydrocarbon C7H16
IHD = (16-14)/2 = 1
Problem: calculate the IHD for niacin, molecular formula C6H6N2O
reference hydrocarbon C6H16
IHD = (16 - 6)/2 = 5 O
Isopentyl acetate

10. IUPAC Nomenclature
1. Number the longest chain of carbon atoms that contains the double bond in the direction that gives the carbons of the double bond the lowest numbers 2. Locate the double bond by the number of its first carbon 3. Name substituents 4. Number the carbon, locate and name substituents, locate the double bond, and name the main chain

11. Common Names
Despite the precision and universal acceptance of IUPAC nomenclature, some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

12. Common Names
the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups

13. The Cis,Trans System
Configuration is determined by the orientation of atoms of the main chain C H 3 ( ) 2 c i s - , 4 D m e t h y l p n 1 r a x

14. The E,Z System uses priority rules
if groups of higher priority are on the same side, the configuration is Z (German, zusammen)
if groups of higher priority are on opposite sides, the configuration is E (German, entgegen)

15. Cis,Trans Isomerism Cycloalkenes
in small-ring cycloalkenes, the configuration of the double bond is cis
these rings are not large enough to accommodate a trans double bond

16. Cis,Trans Isomers
1,2-Dimethylcyclopentane

17. Cis,Trans Isomerism
1,4-Dimethylcyclohexane

18. Cis,Trans Isomerism
trans-cyclooctene is the smallest trans cyclooctene that has been prepared in pure form and is stable at room temperature
the cis isomer is 38 kJ (9.1 kcal)/mol more stable than the trans isomer
the trans isomer is chiral even though it has no chiral center

19. Dienes, Trienes, and Polyenes
For alkenes containing two or more double bonds, change the infix -en- to -adien-, -atrien-, etc.
those containing several double bonds are often referred more generally as polyenes
following are three dienes

20. Dienes, Trienes, and Polyenes
for alkenes with n double bonds, each of which can show cis,trans isomerism, 2n stereoisomers are possible
example: 22 = 4 cis,trans isomers are possible for 2,4- heptadiene

21. Dienes, Trienes, and Polyenes
vitamin A, a biologically important compound for which a number of cis,trans isomers is possible
there are four double bonds about which cis,trans isomerism is possible, for 24 = 16 stereoisomers

22. Physical Properties Alkenes are nonpolar compounds
The only attractive forces between their molecules are dispersion forces
The physical properties of alkenes are similar to those of alkanes

23. Terpenes
Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

24. Terpenes
Myrcene, C10H16, a component of bayberry wax and oils of bay and verbena
Menthol, from peppermint

25. Terpenes
-Pinene, from turpentine
camphor, from the camphor tree

26. Fatty Acids
Animal fats and vegetable oils are both triesters of glycerol, hence the name triglyceride
hydrolysis of a triglyceride in aqueous base followed by acidification gives glycerol and three fatty acids
fatty acids with no C=C double bonds are called saturated fatty acid
those with one or more C=C double bonds are called unsaturated fatty acids

27. Fatty Acids
the most common fatty acids have an even number of carbons, and between 12 and 20 carbons in an unbranched chain
the C=C double bonds in almost all naturally occurring fatty acids have a cis configuration
the greater degree of unsaturation, the lower the melting point
triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils
triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats

28. Fatty Acids
carbon chains of saturated fatty acids exist largely in the staggered, anti-conformation
because of their high degree of order, they pack together well and are held together by dispersion forces
as a result both saturated fatty acids and triglycerides derived from them are solids at room temperature
following is a saturated triglyceride

29. Fatty Acids
cis double bonds place kinks in the chains of unsaturated fatty acids
unsaturated fatty acids and the triglycerides derived from them do not pack as well in a crystal lattice as their saturated counterparts, and have weaker dispersion forces between their molecules
butter fat, for example, has a high content of saturated fatty acids and is a solid at room temperature
salad oils (from plant oils) have a high content of polyunsaturated fatty acids and are liquid at room temperature