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Solution Manual For Organic Chemistry Second Edition Jonathan Clayden, Nick Greeves, and Stuart Warren
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Organic Chemistry I
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Chemistry of the Carbonyl Group
16 October 2017 18:02
PART 1: CHEMISTRY OF THE CARBONYL GROUP: THE BASICS Carboxylic Acid Derivatives Cyclic Esters and Amides
• In the case of aldehydes the hydrogen substituent enables Acid chlorides and acid anhydrides are Cyclic amide Cyclic ester
further oxidation to the corresponding carboxylic acid. very reactive electrophiles so are
susceptible to hydrolysis, reforming
the carboxylic acid.
Oxyanion structure
Treating a Carboxylic Acid with Base preferred. Structure of the Carbonyl
Group using a Molecular-
Orbital Approach
In the carbonyl group the carbon and oxygen
An enolate is a atoms are both sp2 hybridised.
• Carboxylic acid derivatives can form enols of some kind. Those of conjugate base of enol. • Carbon atom has 3 sp2 HAOs and one p
esters are particularly important and either enols or enolates are The negative charge is AO to form a π-bond
easily made. Necessary to avoid water in the presence of acid or mainly on oxygen, most • Likewise for oxygen although two of the
base, as esters hydrolyse under these conditions. Can use alkoxide electronegative atom. sp2 have been filled, each with two
belonging to the ester to make enolate ions. electrons (these are lone pairs)
π-Framework: • The p AO on each atom is orthogonal to the plane containing the 3 sp2 HAOs.
• p AOs can also overlap to form a bond, however this side-on orbital overlap is less • Therefore the bonding structure of the carbonyl group can be divided into
effective than direct orbital overlap hence results in formation of a weaker π-bond. the σ-bonding framework and π-bonding framework.
• As before two overlapping p AOs from two MOs, but this time a lower-energy bonding σ-Framework:
π-MO and a higher-energy antibonding π*-MO.
• Formation of a σ-bond involves the direct overlap of two orbitals. C-O σ-
bond is formed by direct overlap by one sp2 HAO from carbon and one
sp2 HAO from oxygen. When they overlap they generate two new MOs.
• Zero probability of the electrons in the π-MO existing in the plane containing the σ- Shape of the σ and σ* MOs:
bonding framework (nodal plane). Electron density in the π-bond therefore lies above
and below the plane containing the σ-bonds.
• One electron in each overlapping HAO so the two new MOs are filled
with these two electrons, with the lowest energy orbital being filled first.
Since an orbital (AO, HAO or MO) can accommodate up to two electrons,
with two electrons the lower-lying bonding σ-MO is completely filled
leaving the higher-energy σ*-MO empty. System generated is overall
Can see the energy of both systems described by overlaying the two MO energy diagrams
lower in energy.
When a carbonyl group reacts with a • Two remaining sp2 HAOs on the carbon atoms form similar σ-bonds to
nucleophile, electrons move from the the atoms either side of the carbonyl group by overlapping with orbitals
HOMO of the nucleophile (an sp orbital on these adjacent atoms e.g. in acetaldehyde, orbital overlap with an sp3
in the case of cyanide) into the LUMO of HAO on the adjacent carbon and a 1s AO on the adjacent hydrogen.
the electrophile (π* orbital of C=O bond). • On oxygen two remaining sp2 HAOs on oxygen are each filled with two
As the Nu- approaches the carbon atom, electrons, these represent lone pairs.
electron pair in HOMO starts to interact
with the LUMO to form a new σ-bond. Polarisation of C=O bond
Filling the π* breaks bond so π-bond • Electronegativity of oxygen on the Pauling scale is 3.44, electronegativity
broken leaving only C-O σ-bond intact. of carbon is 2.55 (there is an uneven distribution of electron density in
The electrons in the π-bond move to C=O).
electronegative oxygen, which ends up • Build-up of positive charge on the electron-deficient carbon end of the
with the negative charge that started on functional group and build-up of negative charge on electron-rich oxygen.
the nucleophile.
• The Highest Occupied Molecular Orbital (HOMO) is the πC=O Molecular Orbital.
• The Lowest Unoccupied Molecular Orbital (LUMO) is the π*C=O Molecular Orbital. • The unfilled π* antibonding orbital is skewed in the opposite direction,
with a larger coefficient at the carbon atom.
Keto-Enol Tautomerism Consequences of a polarised C=O bond:
• In order to interconvert keto and enol tautomers, move a proton from one of the alpha 1. As a consequence of a polarised bond, carbonyl
carbons on to the carbonyl oxygen and shift the location of the double bond. This type of group exhibits a strong permanent dipole moment.
interconversion is known as tautomerism This has the effect of making the C=O stretching
• The combination of a C=C double bond and O-H bond is slightly less stable than the band in IR spectra very intense.
combination of C=O and C-H bond, so simple aldehydes and ketones do not exist as Dipole moments have magnitude and direction (vectors)
enols. 2. C=O group is thermodynamically more stable than a C=C double bond
• 1,3-dicarbonyl compounds form stable due to ionic character (atoms held together by electrostatic interactions).
enols - unique 1,3 arrangement of two Hence formation of C=O bond often provides a thermodynamic driving
C=O groups leads to conjugated enols force in chemical reactions, and C=O bond distance in a carbonyl group is
shorter (1.22 anstrong) than the C=C bond (1.32)
1. The 1H-NMR and 13C-NMR spectra of (analytically pure) acetylacetone show the
presence of two compounds: 3. Carbonyl groups are electrophilic (i.e. susceptible to attack by
nucleophiles). Nucleophiles are attracted to the partial positive
charge of the carbon atom and repelled by the oxygen atom on
electrostatic grounds.
Therefore react with carbonyl compounds at the carbon centre.
Additions to carbonyl groups generally consist of nucleophilic attack on the carbonyl group and
protonation of the anion that results.
However the 1H-NMR and 13C-NMR spectra of hexane-2,5-dione are consistent with there being a single
compound:
(Just added another CH2 compared to spectra on the left of the page)
• The NMR spectra of acetylacetone
provide a first indication that this
Organic Chemistry I Page 1
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