Chem 351 Fall 2005 Final : Mechanism

Part 7: MECHANISMS

I-i (a) The first step in the SN1 reaction of the alkyl halide to form an alcohol. The better leaving group is the bromide and the more stable carbocation is the tertiary cation - therefore the bromide is lost then water acts as the nucleophile leading to the alcohol. Nitrate is a very poor nucleophile, the water solvent is the reactive nucleophile. The final part is just a simple acid / base reaction to remove the proton leading to the alcohol.

(b) The base, sodium hydroxide (NaOH) then removes the proton from the alcohol to form the alkoxide (which is a better nucleophile due to the negative charge on the oxygen) then the oxygen acts as a nucleophile in an intramolecular SN2 (primary center) displacing the chloride to give the cyclic ether.

intramolecular SN2 to give a cyclic ether

I-ii The reaction that forms the bromo-alcohol from the cyclic ether is a substitution reaction - it's essentially the same as the reaction of alcohols with HBr to give alkyl bromides...alcohol with HBr. The mechanism requires that the ether O be protonated first to make a better leaving group (-ve O groups are poor leaving groups). The C-O to the tertiary carbon breaks to give a tertiary carbocation which are favourable (hence SN1 reaction). Then the bromide ion attacks to give the tertiary bromide.

Notes: Common errors in this question:

i (part a) using SN2 despite the SN1 conditions, making very very unstable dications, loss of the Cl first (it's the poorer leaving group), not using H2O as the nucleophile to give the intermediate alcohol.
(part b) not making the alcohol into the alkoxide using the base, using SN1 despite the primary carbocation

ii not protonating the O to make a better leaving groups, using SN2 when it's SN1

II-i The reaction that forms di-n-propyl ether from 1-propanol is a substitution reaction. The reagent, dilute sulfuric acid helps favour substitution over elimination (only some of the alcohol is protonated at any one tome so there is always ROH present to act as the nucleophile. The mechanism requires that the alcohol O be protonated first to make a better leaving group (-ve O groups are poor leaving groups) and then the oxygen of propanol attacks to displace the leaving group in an SN2 type reaction. It will not react as SN1 because primary carbocations are too unfavourable. The sulfate or bisulfate ion is a very poor nucleophile as the -ve charge is resonance stabilised. The final step is just an acid / base reaction to remove the proton :

II-ii The reaction that forms propene from propanol is an elimination reaction. The sulfuric acid, is a strong acid and alcohols that give stable carbocations tend to undergo E1 eliminations. However, in this case we would get a very unfavourable primary carbocations so we are looking at an E2 reaction : concerted reaction to remove the proton, form the C=C and loss of water molecule.

III-iii

Cyclohexanol is reactive to both nucleophile substitution and elimination while phenol is not. Consider each of the possible mechanisms...

SN1 and E1: These reactions have a common rate determining step, carbocation formation. The phenol would require the formation of a very unfavourable phenyl cation. Hence phenol doesn't undergo Sn1 or E1 reactions.

SN2 : This requires backside attack. The planar nature of the phenol prevents this. Cyclohexanol is not planar, the OH is on a tetrahedral, sp3 C, so the nucleophile can attack.

E2 : These eliminations require that the H and the LG be coplanar, most commonly anti (180 degrees) but syn (0 degrees) is an alternative. The phenol could react via a syn elimination to give benzyne (see above) - but this is a very reactive compound (but it is known).

Notes: Common errors in this question:
i not protonating the O to make a better leaving groups, using SN1 when the primary system means it will be SN2, using HSO₄- or H₂SO₄ as a base (?), forming and using RO- as the nucleophile despite the acidic conditions.
ii not protonating the O to make a better leaving groups, using E1 when it's E2 since the primary carbocations are very unfavourable.
iii referring to breaking the octet rule in the phenyl system (benzyne is the possible elimination product).