CHEM1112 Lecture Notes - Lecture 6: Nucleophilic Addition, Sigma Bond, Pi Bond
17 May 2018
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• Predict the products of oxidation and reduction of aldehydes and ketones and give the reagents
required to perform these reactions
• Carbonyl: C=O. electron density above and below a plane. Carbonyl bond= isoelectronic meaning
electrons are identical in their arrangement, sigma between Carbon and Hydrogen. Pi bonds are
weakly held, 2 unhybridised p orbitals overlap poorly and are mobile.
Carbon in carbonyl is a good electrophile
Addition reaction- but difference in electronegativity of C and O, mechanism is nucleophilic addition,
strong dipole associated with double bond, polarisation pronounced due to pi electron pairs, carboonyl
vibration strong in IR spectra, changes in dipole moment causes high intensity in IR absorption. First step
in reaction involves nucleophile attracted to electrophilic delta+ end of the carbonyl carbon >
intermediate> picks up a proton> breaks pi bond> new C-nucleophile bond> electron pair in pi bond =
lone pairs.
Charged oxygen in the intermediate product (alkoxide) gives a neutral product, often a proton from
water > work up the reaction to isolate organic product.
Reaction with a hydride: nucleophile, has a lone pair, nucleophilic addition- reduction reaction. Hydride
form C-H sigma bond, break pi bond> intermediate, alkoxide> (2 step process 1: hydride addition, 2: add
aqueous acid) put proton onto Oxygen> alcohol product. Hydride reduction of ketone= secondary
alcohol, reversible through oxidation with acidic dichromate. Driving force of addition reaction is
replacing pi bond with C-H sigma bond- thermodynamically favorable, fast, can occur at low
temperatures.
Common sources of hydrides: metal hydrides, LiAlH4, NaBH4- very strongly bonded, helps push reaction
in the forward direction, metal can be central in pushing reaction forward.
Aldehydes> primary alcohols
Ketones > secondary alcohols
• Give the products from the reaction of a carboxylic acid with a base and recognise and recognise
that it’s a reversile reation
Addition of carbon based nucleophiles: build up complex molecules by making C framework of
molecules bigger.
Grignard Reagents: carbanions used if electron rich C bonds to electropositive metal, C-metal and Metal-
H are very polar and weak bonds, can be used to provide hydrides- good nucleophile, replaced with a C-
C or C-H sigma bond which is more thermodynamically favourable.
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Making grignard reagents: can be made from organohalides and magnesium metal. Bond from C in
Grignard reagent always > metal, not halogen. Gr + CO2 > introducing carboxylic acid, Gr + ketone>
tertiary alcohol, introduce C constituent.
Anhydrous HBr reacted with Mg, replace Bromide> + ketone (gotten using dichromate > tertiary alcohol.
• Predict the product the reduction of a carboxylic acid and give the reagents required to perform
this reaction
Making carboxylic acids: grignard reagent with CO2, oxidation of primary alcohols, oxidation of
aldehydes- strong oxidant oxidises a primary alcohol forming aldehyde which is readily oxidised to a
carboxylic acid.
Aspirin- substituted benzoic acid
Carboxylic acids are weak acids, pKa~5 (low, does not dissociate fully)
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find more resources at oneclass.com
Document Summary
Predict the products of oxidation and reduction of aldehydes and ketones and give the reagents required to perform these reactions, carbonyl: c=o. electron density above and below a plane. Carbonyl bond= isoelectronic meaning electrons are identical in their arrangement, sigma between carbon and hydrogen. Pi bonds are weakly held, 2 unhybridised p orbitals overlap poorly and are mobile. First step in reaction involves nucleophile attracted to electrophilic delta+ end of the carbonyl carbon > intermediate> picks up a proton> breaks pi bond> new c-nucleophile bond> electron pair in pi bond = lone pairs. Charged oxygen in the intermediate product (alkoxide) gives a neutral product, often a proton from water > work up the reaction to isolate organic product. Reaction with a hydride: nucleophile, has a lone pair, nucleophilic addition- reduction reaction. Hydride form c-h sigma bond, break pi bond> intermediate, alkoxide> (2 step process 1: hydride addition, 2: add aqueous acid) put proton onto oxygen> alcohol product.
