Unit 6: Substitution & Elimination Reactions

substitution reaction

when a nucleophile replaces a leaving group in an alkyl halide

elimination reaction

when a molecule reacts with a base and forms a pi bond in an alkyl halide; this occurs through the loss of a β-hydrogen and a leaving group

α (alpha) carbon

the carbon located the closest to the halogen in an alkyl halide; there can only be 1 of these

β (beta) carbon

the carbon located closest to the α (alpha) carbon in an alkyl halide; there can be up to 3 of these

1. primary (1°)

2. secondary (2°)

3. tertiary (3°)

ways to classify alkyl halides based on the number of R groups branching from the alpha carbon/in the beta areas

primary (1°)

an alkyl halide that has 1 R group branching from the α (alpha) carbon

secondary (2°)

an alkyl halide that has 2 R groups brancing from the α (alpha) carbon

tertiary (3°)

an alkyl halide that has 3 R groups brancing from the α (alpha) carbon

Sn2 reaction

a type of substitution reaction where a nucleophilic attack and a loss of a leaving group occur simultaneously; is a one-step reaction with no intermediate

• strong nucleophiles are required for these reactions

• the nucleophile will be added with the opposing stereochemistry to the leaving group

rate = k[alkyl halide][nucleophile]

the rate law equation for a Sn2 reaction where 2 molecules are determining the reaction rate

Sn1 reaction

a type of substitution reaction where a loss of a leaving group occurs followed by a nucleophilic attack with a carbocation intermediate; is a two-step reaction

• weak nucleophiles are used for these reactions

• carbocation intermediates can rearrange and can be attacked from either side

rate = k[alkyl halide]

the rate law equation for a Sn1 reaction where 1 molecule determines the reaction rate

solvolysis

Sn1 reactions where a proton transfer occurs at the end

1. polar protic solvents

2. polar aprotic solvents

the 2 types pf reaction solvents

polar protic solvents

reaction solvents which HAVE oxygen hydrogen bonds (O-H) or nitrogen hydrogen bonds (N-H); Sn1 reactions are faster in these types of solvents

polar aprotic solvents

reaction solvents which do NOT have oxygen hydrogen bonds (O-H) or nitrogen hydrogen bonds (N-H); Sn2 reactions are faster in these types of solvents

alkene stability

the more substituted/the more substitutents an alkene is/has the more stable it is; cis alkenes are less stable than trans alkenes due to steric interference

E2 reaction

a concerted (occuring simultaneously) proton transfer with a loss of leaving group; the relative size of the base will determine the major product produced

rate = [alkyl halide][base]

the rate law equation for an E2 reaction where 2 molecules determine the reaction rate

small bases

bases which will make the more substituted alkene the major product

bulky bases

bases which will make the less substituted alkene the major product

stereoselective reactions

reactions which have 1 starting material which can gigve 2 potential products; the emajor product will be the most stable and the trans isomer

Zaitsev’s Rule

E2 reactions are governed by the relative stability of the alkenes