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4.1.3 Alkenes

Alkenes

Unsaturated hydrocarbon containing a σ bond (overlap of orbitals directly between the bonding atoms) and a π bond (sideways overlap of adjacent p-orbitals above and below the bonding C atoms) overlapping.

Shape of alkenes – Trigonal planar shape with 120˚ bond angle. Electron pairs repel. Alkenes contain three bonding regions and no lone pairs of electrons.

The π bond causes restricted rotation because it overlaps the σ bond and rotation would disrupt the π bonding system. This means that there is no rotation of the C=C double bonds due to the rigidity of the π bond.

Stereoisomerism – compounds having the same structural formula but with a different arrangement of atoms in space.

E/Z isomerism with multiple identical groups – Z isomers represent an alkene where the same groups are both found above or below the double bondE isomers represent an alkene where the same groups are found on opposite sides of the double bond. 

E/Z isomers where all groups are different – atoms with a higher atomic number have priority. if there are groups, you may have to look along the chain to find which group has priority e.g. a methyl group has 1C bonded to 3H but an ethyl group has 1C bonded to 2H and 1C which is bonded to 3H. At the atom where the two groups differ, the atomic number of that atom is used to determine the priority, once again with a higher atomic number giving higher priority so in this case it is the ethyl group.

(http://www.compoundchem.com/wp-content/uploads/2014/05/Geometric-isomers.png)                          

cis/trans isomerism – a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same.

The use of E as equivalent to trans and Z as equivelant to cis is only consistantly correct when there is a H on each carbon atom of the C=C bond.

Addition reactions of alkenes

Reactivity of alkenes – alkenes are very reactive because the π bond has a relatively low bond enthalpy so it requires little energy to break. For this reason, it is very susceptible to attack from an electrophile (electron pair acceptor).

Adding H2:      The hydrogen adds across the double bond to form an alkane. this reaction requires a Ni catalyst and 150°c

Producing a dihaloalkane:

(http://alevelchem.com/img/electrophilic_addition_bromine.gif)

Reactions of alkenes – steam hydration to form alcohols.

Alkenes can be hydrated by steam at 300°c and 65atm with a phosphric acid catalyst (H3PO4). The water is added across the double bond. The reaction is reversible so the yield can be low but the alkene can be recycled to increase the overall yield.

Reactions of alkenes – addition of hydrogen halides.

Alkenes undergo addition reactions of hydrogen halides to form haloalkanes. The hydrogen halides (e.g. HBr) adds across the double bond. If the alkene is unsymmetrical then there are two products formed. the amount of each product depends on how stable to carbocation formed in the intermediate step is. The stability depends on whether the carbocation is primary, secondary or tertiary (referring to how many alkyl groups surround the carbocation). The more alkyl groups attached, the more stable meaning tertiary carbocations are the most stable and primary carbocations the least. the stability is caused by the electron donation from surrounding carbon atoms towards the positive charge.

Markownikoff’s rule is that the major product from the addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached to it.

Electrophilic addition mechanism:

This mechanism is relevant for every electrophilic addition reaction of alkenes (hydrogen halides, hydrogen, steam, halogens).

(http://www.swotrevision.com/pages/alevel/chemistry/images/img_46.GIF)

Always include dipoles, charges and curly arrows. The curly arrows represent the movement of a pair of electrons.

Polymers – the double bonds in alkenes can open up and join together to form long chains called polymers. The individual units are called monomers.

1 repeat unit looks like this:

(http://www.gcsescience.com/repeat-unit.gif)

Polymerisation is where the alkene undergoes an addition reaction with itself, forming the long chain of repeat units :

(http://www.chemhume.co.uk/ASCHEM/Unit%202/Ch11%20Alcohols/images/polymerisation_chloroethene.jpg)

The problem with polymers is that they are so unreactive that they are not biodegradable. Therefore they are disposed of on landfill sites and by reusing them through cracking techniques, melting and remoulding them or burning them (produces toxic waste products that have to be passed through ‘scrubbers‘ which neutralise them).

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