OChem Chp. 7 - Elimination and Synthesis Reactions

E naming

An alkene with more than two attached groups means the highest priority groups from either side are on opposite sides of the molecule

Z naming

An alkene with more than two attached groups means the highest priority groups from either side are on the same side of the molecule

E2 Reactions

Elimination reactions that involve strong bases and leaving groups, leaving an alkene behind. Happens in one concerted step that is second order. Either Zaitsev or Hoffman and will be Anti-peri planar. The bases remove a hydrogen from an adjacent carbon, and with the leaving group removed, the compound forms an alkene.

Zaitsev’s rule

The tendency of small bases involved in removing hydrogens to attack hydrogens from more substituted carbons, creating an alkene with more substituted groups.

Hoffman’s rule

Large bases involved in removing hydrogens tend to attack carbons with the fewest substituents, as they often have more room. This will form an alkene with the least number of substituted groups on one side.

Anti-peri planar

The requirement that removed groups from a compound must be in the same plane on opposite sides of the molecule. Single bonds will spin to accommodate a favorable Zaitsev reaction.

Small bases

O-, MeO-, EtO-

Large bases

TbuO-

E1 reaction

A first-order elimination reaction in which the leaving group is removed, forming a carbocation, and then a nucleophile removes a hydrogen from an adjacent carbon following Zaitsev’s rule, forming an alkene. This forms a carbocation intermediate and is preferred at higher temperatures, with higher types of carbons (tertiary, secondary) and weaker bases.

Acid Dehydration

An elimination reaction involving Hydroxide. Since Hydroxide is not a good leaving group, protonating it using a strong acid will create water, a good leaving group

Secondary+ acid Dehydration

Engage in a E1 reaction. For this reaction, the water then comes back and removes the hydrogen, acting as both the nucleophile and the leaving group. This reaction mechanism will only occur in a secondary or tertiary carbon

Primary acid dehydration

On a primary carbon, alcohols will be made into water, and then elimination will occur in a concerted E2 reaction, not forming a carbocation

Methyl shift

Methyl group shifts to stabalize a carbocation

Hydride shift

A hydrogen atom shifts to stabalize a carbocation

Ring expansion

A ring will break a bond and form a larger ring to stabilize the angular strain and carbocation. More likely in propyl and butyl rings, as the angle strain is the worst in those; 5-membered rings need to have a high amount of instability to expand

Chain expansions

A reaction with an alkyne and Sodium Amide will allow an alkyne to make a new bond with a given carbon group. That carbon group needs to have a primary leaving group that will attach through an SN2 reaction

Vicinal Dibromide

Dibromide will attach across an alkene through a stepwise, anti-addition reaction

Double elimination

An Alkyne synthesis reaction involving compounds with 2 alkyl halides uses 2 equivalents of amide to remove hydrogens as the bromines leave in an E2 reaction

Hydrogenation

Adding Duterium to alkenes to make alkanes, will release more energy the more stable the alkene was (4-3-terminal-trans-cis-1-methyl). Can look like Pt H2, Pd/C, ect.

Lindlar’s catalyst

Will cause alkynes to convert to cis alkenes when reacted with deuterium and the alkyne

Non-lindlar’s catalysts

Cation-ammonia compounds that will convert an alkyne into a trans alkene also include methyl iodide

PCl5

Will remove a oxygen from a carbonyl, leaving 2 chloride bonds in its place, which can also undergo double elimination