Cycloalkanes Alkanes that form rings are called cycloalkanes

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.

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): 2 C 2 H 6(g) + 7 O 2(g) 4 CO 2(g) + 6 H 2 O(l) H = -2855 k. J C 2 H 6 + O 2 CO + H 2 O (incomplete combustion)

• Substitution – halogen atoms replace hydrogen atoms (UV light is used to break Bonds) CH 4 + X-X CH 3 X + H-X step 1 – initiation (formation of free radical by homolytic fission): Cl 2 2 Cl. (by UV light) step 2 - propagation Cl. + CH 4 CH 3. + HCl CH 3. + Cl 2 CH 3 Cl + Cl. step 3 – termination Cl. + Cl. Cl 2 Cl. + CH 3 Cl CH 3. + CH 3. C 2 H 6 • Dehydrogenation – H 2 is removed, causing a C=C bond CH 3 -CH 3 CH 2=CH 2 + H 2

Unsaturated Hydrocarbons Alkenes • Alkenes contain C, H atoms and single and double bonds. • The simplest alkenes are H 2 C=CH 2 (ethene) and CH 3 CH=CH 2 (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.

• 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

Examples – 1, 2 -dichloroethane Cis – mp = 60. 3ºC trans – mp = 47. 5ºC But-2 -ene cis – mp = -139ºC trans – mp = -106ºC

Examples – name the following 1. transpent-2 -ene 2. 5 -methyl-cis-hex-2 -ene

• • 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.

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.

• A common addition reaction is hydrogenation: CH 3 CH=CHCH 3 + H 2 CH 3 CH 2 CH 3 • 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: CH 2=CH 2 + HBr CH 3 CH 2 Br CH 2=CH 2 + H 2 O CH 3 CH 2 OH

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.

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

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

Polymers • 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

• 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.