Chapter 13: Alkenes (13.1-13.5)

unsaturated hydrocarbons containing the C=C functional group can have multiple double bonds either straight chain, branched, or in rings

CnH2n

consists of a π-bond (pi-bond) and a σ-bond (sigma bond) the π-bond prevents rotation around the C=C bond trigonal planar shape around it, bond angle 120

pi-bond is the sideways overlap of adjacent P orbitals above and below the bonding atoms In the middle of the overlapping orbitals is the sigma bond (overlap of orbitals directly between the bonding atoms)

because alkenes have a π-bond The pi-bond induces a region of high electron density above and below the plane bonding the C atoms in the double bond. Electrophiles can attack the high electron density of the π-bond, causing the alkene to react.

• E/Z isomerism (only occurs in some alkenes) - optical isomerism (occurs in a range of different compounds)

• must have a C=C double bond - must have 2 different groups attached to each carbon atom of the C=C bond

special kind of E/Z isomerism, where both of the C atoms in the C=C bond have 1 of the same group attracted to it. cis: the repeated group is on the same side of both C atoms trans: the repeated group is on the opposite side of both C atoms

the atom with the greater atomic number has the higher priority

a Z isomer has both of the higher priority groups on the same side pf the C=C bond (Zame Zide)

an E isomer has the higher priority groups on the opposite side of the C=C bond

keep going along the chains until you find a difference

a small molecule adds across the double bond, causing the π-bond to break and new σ-bonds to form there is only 1 product the double bond is lost, product is saturated

alkane addition reaction, needs a nickel catalyst

dihaloalkane addition reaction, no catalyst

unsaturated hydrocarbons decolourise bromine water: • orange bromine water is added to an excess of aqueous organic compound (mixture is shaken) • in the presence of a double bond, the mixture becomes colourless (bromine adds across the double bond) • if the organic compound is saturated, no reaction will occur and bromine water will not decolourise

haloalkane (addition reaction, no catalyst) if the alkene has a carbon chain of 3 or more, there are 2 possible products, depending on which carbon the halogen adds to (e.g. 1-bromo vs 2-bromo)

alcohol (addition reaction, acid catalyst e.g. H3PO4) if the alkene has a carbon chain of 3 or more, there are 2 possible products, depending on which carbon the -OH adds to

electrophilic addition

step 1: • a molecule (e.g. H-Br) acts as an electrophile, accepting the pair of electrons from the pi-bond of C=C • the electrophile bond (e.g. H-Br) breaks by heterolytic fission step 2: - a carbocation and negative ion (e.g. Br-) are formed step 3: - the negative ion (Br-) is attracted to the carbocation, forming a bond (C-Br). The product is formed

the high electron density of the pi-bond induces a dipole in the non-polar molecule so it can now act as an electrophile

Markownikoff’s rule states that when an electrophile H-X reacts with an unsymmetrical alkene, the H atom is more likely to attach to the carbon atom with the greatest number of H atoms attached to it i.e. secondary carbocation will form the major product

they are much more stable without the C=C double bond

addition polymers are unreactive and take a long time to biodegrade