Properties and reactions of Alkenes

Alkenes Alkenes are hydrocarbons with one or more double bond. An alkene only needs one double bond to be an alkene. Alkenes are unsaturated. This means that there are some bonds that can reform to absorb more hydrogen. If a hydrocarbon has many double bonds it is polyunsaturated (poly = many)

Geometric isomers The double bond of an alkene cannot be rotated. This means that there can be different forms of molecules with two functional groups, one of either side of the bond.

Where the functional groups are on the same side of the molecule it is shown by putting cis in the name. Where the functional groups are on different sides of the molecule it is named trans. Cis 1,2 –di-chloro-ethene Trans 1,2 –di-chloro-ethene

Preparation of alkenes Alkenes can be formed by cracking of alkanes Alkenes can be formed by the dehydration of alcohol

Reactions of Alkenes Addition reactions occur when other atoms are added to the molecule. This is called adding across the double bond. The double bond becomes a single bond and two new single bonds are formed

Types of addition reaction Type of reaction Reagent name Reagent symbol Product Family Hydrogenation Hydrogen Pt/H2 or Ni/H2 alkane Hydration Water H+/H2O alcohol Halogenation Halogen Br2 or Cl2 haloalkane Hydrogen halide HBr or HCl Oxidation Potassium permanganate H+/MnO4- diol

Markovnikov’s Rule In addition reactions with HX (HBr, HCl, HI) or water there are two options for the placement of the two atoms across the molecule. In these cases the rule is “the rich get richer” Or The carbon with the most H atoms will get more. The carbon with the fewest H atoms will get the X (Br, Cl or I) or the OH- group

Haloalkanes Haloalkanes are polar and have higher melting and boiling points than their alkanes. Even though they are polar they do not mix with water Nearly all are solid or liquid at room temperature

Polymerisation A polymer is a large molecule made up o f many small molecules. Monomers are small molecules which join together to make polymers. Poly = many Mono= one Alkenes can join together to in addition polymers where the double bonds break to form a long singly bonded chain

Examples of polymers