Chem 352, wk4-5, Polar Pi bonds

polar pi bond

such as C=O or N=C were the carbon is electron poor and the electronegative atom is electron rich

• a nucleophile comes in and attacks the carbon electrophile

• 1) nuc addition 2) proton transfer

Oxidation State

OIL RIG = oxidation is loss of electrons, reduction is gain

• o.s = valence e - owned e

• C owns one e- from a c-c bond

• C own zero e- from a bond with a more electronegative atom (O)

• C owns two e- from a bond with a less electronegative atom (H)

Reduction of carbonyls

reduce aldehyde to make 1 degree alcohol

reduce ketone to make 2 degree alcohol

using NaBH4 or LiAlH4

LiAlH4 can reduce

aldehydes, ketones, esters and carboxylic acids because it is more electronegative

NaBH4 can reduce

aldehydes and ketones, but not esters and carboxylic acids

Why can’t NaH be used to reduce things?

NaH is a strong base but a poor nuc

• cant be used for addition, makes enolates instead

Organometallic Reagents

organolithium, gringards, lithiumdicuperates

alkyl lithium

Organometallic Reagent that can be used to make new c-c bonds

• highly reactive

• Carbon bonded to lithium has a partial negative charge, making it nucleophilic.

• This property allows them to react readily with compounds like water, alcohols, and carbonyl groups in ketones and aldehydes, forming new carbon-carbon bonds

Grignard Reagents

Mg inserts itself into a carbon-halogen bond

• can add to carbonyls to make alcohols

• can open epoxides to make alcs

• check for chiral centers that make racemic mixtures

Limitations of Gringards

can’t have any water or other acidic protons

can’t have more than one type of electrophillic mulit bond group or it will compete for the reaction

Wittig Reagents

Phosphorus attached to three benzene rings and a negatively charged carbon with two methyl groups

• helps make alkenes but unlike E2 reactions, uses SN2

Reaction Speed of carbonyls

formaldehyde reacts faster than aldehyde which reacts faster than a ketone

• ketones have the most steric hindrance

Organocuperates

also called gilman’s reagents, and lithium dialkylcuprates (R₂CuLi)

• softer nucleophiles than than Grignard or organolithium reagents cus smaller difference in electronegativity

• useful nucleophiles for conjugate addition as well as SN2 reactions

• halide/tosylate couplings, substituted epoxide ring openings, conversion of acid chlorides to ketones

Which reagent(s) will undergo the 1,4 addition to an alpha, beta unsaturated carbonyl?

Cuprates (aka Gilman Reagents)

Which reagent(s) will undergo the direct addition to a polar pi bond?

Organolithium reagents (R-Li), LiAlH4 and Grignard reagents (RMgX)