Addition reactions of alkenes — AQA A-Level Chemistry
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Addition reactions of alkenes explained
When a reagent such as hydrogen bromide adds to an unsymmetrical alkene, two products are possible because H⁺ can attach to either double-bond carbon.
Read the full explanation
Which one dominates is decided by the intermediate, not by the product. The H⁺ adds so as to leave the positive charge on the more substituted carbon, because alkyl groups are electron-releasing: their positive inductive effect pushes electron density towards the carbon and spreads the charge, so a tertiary carbocation is more stable than a secondary one, which is more stable than a primary one. The more stable carbocation forms faster, so the bromide attacks it more often and its product is the major one. The minor product comes from the less stable carbocation.
Students should be able to: outline the mechanisms for these reactions explain the formation of major and minor products by reference to the relative stabilities of primary, secondary and tertiary carbocation intermediates.
The mechanism is electrophilic addition and it runs in two steps. First, the π electrons of the C=C attack the δ⁺ hydrogen of H–Br, or of an O–H bond in sulfuric acid; the arrow starts at the double bond and ends at that hydrogen atom, while a second arrow shows the H–Br or H–O bond breaking, its electrons going to the halogen or oxygen. That gives a carbocation and a negative ion. In the second step an arrow runs from a lone pair on the negative ion to the positively charged carbon, forming the new bond. With bromine the same pattern applies once the non-polar Br–Br bond is polarised by the approaching electron-rich double bond. Choosing where to put the positive charge decides the major product.
Your focus
- Draw both carbocation intermediates possible when a named unsymmetrical alkene reacts with hydrogen bromide.
- Predict the major product and justify the choice by the relative stability of the intermediates.
- Explain, using the inductive effect, why a tertiary carbocation is more stable than a primary carbocation.
Show all 6 objectives
- Draw the full electrophilic addition mechanism for an alkene reacting with hydrogen bromide or with sulfuric acid.
- Explain which carbocation forms preferentially and use it to predict the major product.
- Check a drawn mechanism against the arrow conventions and identify any arrow that would gain no credit.
Addition reactions of alkenes exam tips
Marking Points
- one mark for the structure of the minor product drawn unambiguously
- one mark for saying the major product is formed via the more stable carbocation
- one mark for stating that a tertiary carbocation is more stable than a primary one
- one mark for the electron-releasing character, or positive inductive effect, of the alkyl groups spreading the positive charge
- one mark for naming the mechanism electrophilic addition
- one mark for the arrow from the C=C to the hydrogen atom of the H–Br or H–O bond
- one mark for the arrow showing that H–Br or H–O bond breaking
- one mark for the correct carbocation structure with the positive charge on the right carbon
- one mark for the arrow from a lone pair on the negative ion to the positively charged carbon
Examiner Tips
- 💡Draw both carbocations before choosing a major product; the classification, primary or tertiary, is the argument.
- 💡Say "more stable carbocation" explicitly, then give the inductive effect as the reason; both are separate marks.
- 💡Count alkyl groups on the positive carbon, as AQA credits the comparison of three against one.
- 💡Check every curly arrow starts on a bond or a lone pair; that single rule protects several marks.
- 💡Draw the intermediate large and clear, with the charge next to the correct carbon.
- 💡The arrow forming a bond may stop at the atom, beside it, or more than half way to it, so aim it unmistakably.
Common Mistakes
- comparing the stabilities of the two products rather than of the two carbocation intermediates
- describing alkyl groups as electron-withdrawing, which would reverse the order of stability
- saying "three alkyl groups push electrons" without saying this stabilises the positive charge
- adding the H⁺ to the carbon that already carries fewer hydrogen atoms
- starting the first arrow at a carbon atom instead of at the double bond, which AQA does not credit
- drawing the arrow from H–Br towards the double bond, which reverses the electron flow
- placing the positive charge on the carbon that gained the hydrogen
- showing the negative ion attacking with no lone pair drawn on the attacking atom