1. Cycloalkanes
Alkanes that form rings are called cycloalkanes.
Cyclopropane and cyclobutanes are strained because the C-C-C bond angles in the ring are less than 109.5 required for the tetrahedral geometry.
Because of the strain in the ring, cyclopropane is very reactive.

3. Alkanes do combust in air (making them good fuels):
Reactions of Alkanes
The C-C and C-H bonds are very strong and relatively nonpolar. Therefore, alkanes are very unreactive.
At room temperature alkanes do not react with acids, bases, or strong oxidizing agents.
Alkanes do combust in air (making them good fuels):
2C2H6(g) + 7O2(g)  4CO2(g) + 6H2O(l) H = kJ
C2H6 + O2  CO + H2O (incomplete combustion)

4. Dehydrogenation – H2 is removed, causing a C=C bond
Substitution – halogen atoms replace hydrogen atoms (UV light is used to break Bonds)
CH4 + X-X CH3X + H-X
step 1 – initiation (formation of free radical by homolytic fission): Cl2  2 Cl. (by UV light)
step 2 - propagation
Cl. + CH4  CH3. + HCl
CH3. + Cl2  CH3Cl + Cl.
step 3 – termination
Cl. + Cl.  Cl2
Cl. + CH3.  CH3Cl
CH3. + CH3.  C2H6
Dehydrogenation – H2 is removed, causing a C=C bond
CH3-CH3 CH2=CH2 + H2

5. Unsaturated Hydrocarbons
Alkenes
Alkenes contain C, H atoms and single and double bonds.
The simplest alkenes are H2C=CH2 (ethene) and CH3CH=CH2 (propene):
their trivial names are ethylene and propylene.
Alkenes are named in the same way as alkanes with the suffix -ene replacing the -ane in alkanes.
The location of the double bond is indicated by a number.

6. Geometrical isomers are possible since there is no rotation about a C=C  bond. Geometrical isomers have different physical properties.
Cis = the same side
Trans = across

7. Examples – 1,2-dichloroethane
Cis – mp = 60.3ºC trans – mp = 47.5ºC
But-2-ene
cis – mp = -139ºC trans – mp = -106ºC

8. Examples – name the following1.
transpent-2-ene 2.
5-methyl-cis-hex-2-ene

9. Alkynes
Alkynes are hydrocarbons with one or more CC bond. Therefore, alkynes have one  and two  bonds between two C atoms.
Ethyne (acetylene) is a reactive alkyne: HCCH.
When acetylene is burned in the presence of oxygen (oxyacetylene torch) the temperature is about 3200 K.
Alkynes are named in the same way as alkenes with the suffix -yne replacing the -ene for alkenes.

10. Unsaturated Hydrocarbons Addition Reactions of Alkenes and Alkynes
The most dominant reaction for alkenes and alkynes involves the addition of something (hydrogen, halogen, water) to the two atoms which form the double bond:
Note that the C-C  bond has been replaced by two C-Br  bonds.
Alkenes will react with bromine water by addition, while alkanes will show no reaction with bromine water. This is one way to distinguish between alkanes and alkenes.

11. CH3CH=CHCH3 + H2  CH3CH2CH2CH3
A common addition reaction is hydrogenation:
CH3CH=CHCH3 + H2  CH3CH2CH2CH3
Hydrogenation requires high temperatures and pressures as well as the presence of a catalyst.
It is possible to cause hydrogen halides and water to add across  bonds:
CH2=CH2 + HBr  CH3CH2Br
CH2=CH2 + H2O  CH3CH2OH

12. Mechanism of Addition Reactions
Consider the reaction between 2-butene and HBr:
Careful kinetics experiments show the rate law to be
Therefore, both 2-butene and HBr must be involved in the rate determining step.

13. From the kinetics data, we can propose the following mechanism:
The p-electrons in the alkene attack the δ+ H atom of the HBr to leave a positive charge on one carbon (slow step):

14. Then the pair of electrons on bromide attacks the carbon with a positive charge to give the product.

15. Polymers are large chains built from smaller molecules (monomers)
Addition Polymerization (ex. – polyethene)
Initiation – free radical is formed (species with an odd electron)
Propagation – happens over and over again – chain becomes very long

16. Termination – 2 radicals combine
Economic Importance of alkene reactions:
Hydrogenation of vegetable oil used in the manufacture of margarine.
Hydration of ethene used in the manufacture of ethanol
Polymerization used in the manufacture of most plastics.