Topic 2 Alkenes and alkynes lassification of rganic Reactions Four basic types of organic reaction Acid base reactions Substitution reactions Addition elimination reactions xidation reduction reactions Two types of reagent that participate in reactions Electrophiles (seek electron pairs) eg, l, N 2 Nucleophiles (donate electron pairs) eg -, Br -, :N 3 A reaction may be classified in terms of the overall change to the organic substrate and the nature of the attacking reagent urly arrow notation urly arrow indicates electron pair movement A curly arrow starts at the electron pair that moves and ends at the atom to which the electron pair has moved 2 An arrow from a bond indicates breaking of that bond; an arrow between two species indicates a bond is formed between the species R R 2 20
Electrophilic Addition Reactions Structure of alkenes A double bond is composed of one σ and one π bond The electron density of a π bond is above and below the plane of the double bond - bond - bond all six atoms lie in one plane Reactions of alkenes Dominated by weak and accessible π-bond Electrophiles attracted to the π bond Addition occurs with replacement of a π bond with a σ bond: energetically favourable A B A B A-B could be 2, l 2, Br 2, l, Br, I, 2 1. ydrogenation (addition of hydrogen) Examples: 3 Pd/ catalyst 3 3 3 2. alogenation (addition of a halogen) 3 3 Br Br Br Br 3. ydrohalogenation (addition of a hydrogen halide) 3 3 3 3 Br Br 4. ydration (addition of water) 3 3 3 2 S 4 catalyst 3 21
ydrohalogenation - mechanism Reaction occurs in two steps An unstable intermediate is formed 3 3 Br step 1 3 3 Br nucleophile nucleophile electrophile a electrophile step 2 alogenation - mechanism Br 3 3 The approaching halogen is polarised by the alkene and so acts as an electrophile 3 3 l l 3 3 l l nucleophile electrophile ydration - mechanism 3 3 induced dipole in l 2 l 3 l 3 Addition of water catalyst is required dilute 2 S 4 is used A third step is involved step 1 3 3 step 2 water molecule 3 3 step 3 3 3 electrophile nucleophile 22
Question: Draw the products I Br 2 Selectivity dil 2 S 4 l l l (Z)-2-butene 2-chlorobutane l l l 2-methyl-2-butene 3-chloro-2-methylbutane 2-chloro-2-methylbutane Markovnikov s Rule l l l Minor product Major product Markovnikov s Rule: The hydrogen of an unsymmetrical reagent adds to the carbon of the double bond that has the greatest number of hydrogen atoms already directly attached to it 23
arbocations The intermediate in the reaction is a arbocations have 6 electrons and a positive charge All s are unstable but not all to the same extent arbocation stability 3 3 3 3 3 3 a tertiary alkyl a secondary alkyl a primary alkyl a methyl more stable least stable Markovnikov s rule explained The major product results from the more stable intermediate 3 l 3 3 step 1 3 3 3 l minor product l 3 3 3 less stable 3 3 3 more stable l 3 3 3 l major product Question: Draw the major products of the following reactions 24
3 3 3 2 / (dil 2 S 4 ) Br I ydrogenation atalyst required (eg Pd on charcoal) to break strong - bond 3 3 3 3 2 Pd/ 3 3 3 3 2 Pd/ 3 3 2 Pd/ 3 3 Alkynes A triple bond is composed of two π- and one σ-bond Electrophilic addition occurs to both π-bonds Markovnikov s Rule applies to terminal alkynes 3 3 excess Br 2 3 Br Br 3 Br Br l excess l l excess 2 Pd/ 25
Question: Fill in the boxes: dil 2 S 4 2 / Pd catalyst Br Br Br Summary You should now be able to Understand the mechanism of electrophilic addition Predict the product of an addition reaction to an alkene or alkyne Apply Markovnikov s rule in appropriate cases 26