This document provides an introduction to organic chemistry concepts related to alkenes and alkynes. It discusses the structure and bonding of alkenes and alkynes, including sp2 and sp3 hybridization. It also covers nomenclature rules for naming alkenes and alkynes according to IUPAC conventions. In addition, it discusses the preparation of alkenes and alkynes through elimination reactions like dehydrohalogenation and dehydration, as well as common physical properties.

1. ORGANIC CHEMISTRY I
Alkenes & Alkynes: Introduction
Dra. Sri Sutji Susilowati, M.Si., Apt
M. Salman Fareza, M.Si.
Pharmacy Departement
Jenderal Soedirman University

2. Alkenes
Introduction: Structure and Bonding
• Alkenes are also called olefins.
• Alkenes contain a carbon—carbon double bond.
• Terminal alkenes have the double bond at the end of
the carbon chain.
• Internal alkenes have at least one carbon atom bonded
to each end of the double bond.
• Cycloalkenes contain a double bond in a ring.

3. Alkenes
Introduction: Structure and Bonding
• Recall that the double bond consists of a  bond and a
 bond. The  bond is stronger than the  bond.
• Each carbon is sp2 hybridized and trigonal planar, with
bond angles of approximately 120°.

4. Alkenes
Introduction: Structure and Bonding
• Cycloalkenes having fewer than eight carbon atoms
have a cis geometry. A trans cycloalkene must have
a carbon chain long enough to connect the ends of
the double bond without introducing too much strain.
• trans-Cyclooctene is the smallest isolable trans
cycloalkene. It is considerably less stable than cis-cyclooctene,
making it one of the few alkenes having
a higher energy trans isomer.

5. 5
Alkenes
Introduction: Structure and Bonding

6. Alkenes
Nomenclature of Alkenes:

7. Alkenes
Nomenclature of Alkenes:

8. Alkenes
Nomenclature of Alkenes:
• Always choose the longest chain that contains both
atoms of the double bond.
• Compounds with two double bonds are named as
dienes by changing the “-ane” ending of the parent
alkane to the suffix “–adiene”. Compounds with
three double bonds are named as trienes, and so
forth.
CH2=CH-CH=CH2 CH2=CH-CH=CH-CH=CH2
1,3-butadiene 1,3,5-hexatriene

9. 9
Alkenes
Nomenclature of Alkenes:
• In naming cycloalkenes, the double bond is located between C1 and C2, and the
“1” is usually omitted in the name. The ring is numbered clockwise or
counterclockwise to give the first substituent the lower number.
• Compounds that contain both a double bond and a hydroxy group are named as
alkenols and the chain (or ring) is numbered to give the OH group the lower
number.

10. 10
Figure 10.1 Naming an
alkene in which the
longest carbon chain
does not contain both
atoms of the double bond
Figure 10.2
Examples of
cycloalkene
nomenclature
Alkenes
Nomenclature of Alkenes:

11. 11
Alkenes
Nomenclature of Alkenes:

12. 12
Alkenes
Nomenclature of Alkenes:

13. 13
Alkenes
Nomenclature of Alkenes:
• Some alkene or alkenyl substituents have common names.
• The simplest alkene, CH2=CH2, named in the IUPAC system as ethene,
is often called ethylene.
Figure 10.3 Naming alkenes with common substituent names

14. • Most alkenes exhibit only weak van der Waals interactions, so their
physical properties are similar to alkanes of comparable molecular weight.
• Alkenes have low melting points and boiling points.
• Melting and boiling points increase as the number of carbons increases
because of increased surface area.
• Alkenes are soluble in organic solvents and insoluble in water.
• The C—C single bond between an alkyl group and one of the double bond
carbons of an alkene is slightly polar because the sp3 hybridized alkyl
carbon donates electron density to the sp2 hybridized alkenyl carbon.
14
Physical Properties:
Alkenes

15. 15
Alkenes
Physical Properties:
• A consequence of this dipole is that cis and trans isomeric alkenes often
have somewhat different physical properties.
• cis-2-Butene has a higher boiling point (4°C) than trans-2-butene (1°C).
• In the cis isomer, the two Csp
3—Csp
2 bond dipoles reinforce each other,
yielding a small net molecular dipole. In the trans isomer, the two bond
dipoles cancel.

16. 16
Interesting Alkenes:
Alkenes
Figure 10.4 Ethylene, an industrial starting material for many useful products

17. 17
Preparation of Alkenes:
Alkenes
• Alkenes can be prepared using elimination reactions:
1. Dehydrohalogenation of alkyl halides.
2. Dehydration of alcohols.

18. • Remember, these elimination reactions are regioselective and stereoselective, so the
most stable alkene is usually formed as the major product.
18
Alkenes
Preparation of Alkenes:

19. 19
Alkenes
Introduction to Addition Reactions (see also Chapt. 6):
• The characteristic reaction of alkenes is addition: the  bond is broken
and two new  bonds are formed.
No pi bond
• Alkenes have exposed electrons, with the electron density of the  bond
above and below the plane of the molecule.
• Because alkenes are electron rich, simple alkenes do not react with
nucleophiles or bases, reagents that are themselves electron rich. Alkenes
react with electrophiles.

20. • Because the carbon atoms of a double bond are both trigonal planar, the
elements of X and Y can be added to them from the same side or from
opposite sides.
20
Alkenes
Introduction to Addition Reactions:

21. 21
Alkenes
Introduction to Addition Reactions:
Figure 10.8 Five addition reactions of cyclohexene
No pi bond in
products

22. 22
Alkynes
Introduction—Structure and Bonding:
• Recall that the triple bond consists of 2  bonds and 1  bond.
• Each carbon is sp hybridized with a linear geometry and bond angles of
1800.

23. 23
Alkynes
Introduction—Structure and Bonding:
• Estimation of the energy of pi bonds in ethylene (one  and one  bonds) and
acetylene (one  and two  bonds).

24. 24
Alkynes
Introduction—Structure and Bonding:
• Alkynes contain a carbon—carbon triple bond.
• Terminal alkynes have the triple bond at the end of the carbon chain so that a
hydrogen atom is directly bonded to a carbon atom of the triple bond.
• Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond.
• An alkyne has the general molecular formula CnH2n-2, giving it four fewer hydrogens
than the maximum possible for the number of carbons present. Thus, the triple bond
introduces two degrees of unsaturation.

25. 25
Physical Properties:
Alkynes
• The physical properties of alkynes resemble those of hydrocarbons of similar shape
and molecular weight.
• Alkynes have low melting points and boiling points.
• Melting point and boiling point increase as the number of carbons increases.
• Alkynes are soluble in organic solvents and insoluble in water.

26. 26
Alkynes
Introduction—Structure and Bonding:
• Like trans cycloalkenes, cycloalkynes with small rings are unstable. The carbon
chain must be long enough to connect the two ends of the triple bond without
introducing too much strain.
• Cyclooctyne is the smallest isolable cycloalkyne, though it decomposes upon
standing at room temperature after a short time.

27. 27
Nomenclature:
Alkynes
• Alkynes are named in the same general way that alkenes are named.
• In the IUPAC system, change the –ane ending of the parent alkane name to
the suffix –yne.
• Choose the longest continuous chain that contains both atoms of the triple
bond and number the chain to give the triple bond the lower number.
• Compounds with two triple bonds are named as diynes, those with three are
named as triynes and so forth.

28. 28
Nomenclature:
Alkynes
• Compounds both a double and triple bond are named as enynes. The chain is
numbered to give the first site of unsaturation (either C=C or CC) the lower
number.
• If numbering is equal the ene gets the lower number.
• However, yne is still the parent ending
CH3-CC-CH=CH-CH3 is 2-hexen-4-yne

29. 29
Alkynes
Preparation of Alkyne: Acetylene
• Acetylene can be prepared from calcium carbide and water.
CaC2 + HOH  H-CC-H
• Calcium carbide is prepared by heating coke and calcium oxide in an electric furnace
(~2500o).
C + CaO  CaC2

30. Alkynes
• Recall that alkynes are prepared by elimination
reactions. A strong base removes two equivalents of
HX from a vicinal or geminal dihalide to yield an alkyne
through two successive E2 elimination reactions.
30
Preparation of Alkynes: Dehydrohalogenation
Note: NH3
 NH2
- + H+ ; pKa = ~38 and tBuOH  tBuO + H+ ; pKa = ~19