The mechanism of electrophilic addition reactions describe the mechanism, giving evidence where possible, of: i. the electrophilic addition of bromine and hydrogen bromide to ethene ii. the electrophilic addition of hydrogen bromide to propene Connector: Draw the structural formulae of, and name, the two possible isomers formed when hydrogen bromide reacts with propene. Crowe2008

One might expect that equal amounts of each isomer would be formed, however the major product is always 2-bromopropane

The structure of ethene Draw the molecular orbital diagram for ethene, identifying the type of C-C bonds present. The key to understanding the reactions of the alkenes is that they contain π bonds

Electrophiles The π bond provides a region that is electron rich, above and below the ethene molecule. An electrophile is something which is attracted to electron-rich regions in other molecules or ions. An electrophile is a positively charged species. Electrophile = electron loving

Hydrogen bromide as an electrophile Since the Br atom is more electronegative than the H atom the pair of electrons in the HBr bond are attracted towards the Br atom making it slightly negative (δ-). The H atom therefore becomes slightly positive (δ+) (Note: such a bond is called a polar bond) The slightly positive end of H-Br is attracted to the π bond electrons H H Br Br

Rewrite this mechanism using H-Br for X-Y The mechanism of electrophilic addition of HBr 1. 2. Rewrite this mechanism using H-Br for X-Y Two electrons of π bond form C-X bond X-Y bond breaks, the two electrons move to Y to form a –ve ion Second carbon atom has lost two electrons from π bond and so becomes +ve (known as a carbocation). This attracts the –ve ion, and this donates its two electrons to form a C-Y bond Note: these curly arrows represent two electrons moving

The electrophilic addition of bromine As one bromine atom approaches the ethene molecule, the π electrons repel the pair of electrons in the Br-Br bond and thus polarising the Br2 molecule. This is known as an induced dipole. Br Br Use this information to draw the reaction mechanism for the electrophilic addition of bromine

The electrophilic addition of bromine                                                                                                                                                                               

CARBOCATIONS (or CARBONIUM IONS) A carbocation is a positively charged carbon atom within a molecule     where R = an alkyl group The Alkyl groups tend to "push" electrons away from themselves, causing polarisation of the C-C bond:

Inductive effect of alkyl groups primary < secondary < tertiary The greater the –ve charge directed towards the carbocation, the more stable it becomes: Order of stability of carbocations primary < secondary < tertiary

Propene + HBr – two possible products major product minor product Markovnikov's Rule When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.

Explaining the addition mechanism of HBr and propene secondary carbocation – more stable primary carbocation – less stable Which mechanism has the more stable carbocation? The more stable the carbocation, the faster it forms and so the faster the reaction occurs.