Chapter 13: Alkenes

whilst aliphatic alkenes have the general formula CnH2n, what kinds of alkenes DO NOT?

1. cyclic alkenes

2. alkenes with more than 1 double bond

for every carbon atom of a double, bond 3 are used in …

the other 1 is used in a …

sigma (σ) bonds

pi bond

what is a pi bond?

the sideways overlap of 2 p-orbitals

where is the electron density of a pi bond?

concentrated above and below the line joining the nuclei of bonding atoms

draw the formation of pi bonds:

the shape around each of the carbon atoms in a double bond is:

trigonal planar

1. three regions of electron density

2. three regions repel as far as possible → bond angle is 120^o

3. all atoms in the same plane

what is a stereoisomer?

An isomer with the same structural formula but a different arrangement of atoms in the space

why does E/Z isomerism arise?

• rotation around the double bond is restricted

• groups attached to each carbon are therefore fixed relative to each other

why is a double bond’s rotation restricted?

the position of the ⫪ bond’s electron density is above + below the plane of the sigma bond

what 2 criteria both need to be filled out for E/Z isomerism?

1. carbon=carbon double bond

2. different groups attached to each carbon of the double bond

cis-trans isomerism:

• special case of E/Z isomerism

• one of the atoms on either side of the C=C bond is the same

Cahn-Ingold-Prelog nomenclature:

• if the groups of higher priority are on the same side of the double bond, the compound is a Z isomer

• if the groups of higher priority are diagonally placed across the double bond, the compound is an E isomer

how do we decided what has the highest priority according to Cahn-Ingold-Prelog?

• highest priority atom attached to the carbon has the largest atomic number.

• if they are the same, you will need to find the first point of difference.

why are double bonds more reactive?

• the concentration of electron density is above and below the plane of the sigma bond

• being on the outside of the double bond, the pi electrons are more exposed than the electrons in the sigma bond

• a pi bond readily breaks and alkenes undergo addition reactions relatively easily

calculating the enthalpy of a pi bond:

a C=C bond is…

a pi bond + a sigma bond

hydrogenation of alkenes:

alkene + hydrogen → (over nickel catalyst at 423K ) → alkane

addition reaction def.

2 molecules react together to make 1 product

halogenation of alkenes:

alkene + halogen → dihaloalkane

testing for unsaturation:

1. react with bromine water (orange solution)

2. positive result will turn colourless

3. any compound with a C=C will decolourise bromine water

addition reactions of alkenes + hydrogen halides:

alkene + gaseous hydrogen halide → haloalkane

when an unsymmetrical alkene reacts with an unsymmetrical compound…

two products are possible

hydration reactions of alkenes:

alkenes + steam → (in the presence of a phosphoric acid catalyst, H₃PO₄) → alcohol