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Transcript of THE CHEMISTRY OF ALDEHYDES AND KETONES A guide for A level students KNOCKHARDY PUBLISHING 2008...
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THE CHEMISTRYTHE CHEMISTRYOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES
A guide for A level studentsA guide for A level students
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008
SPECIFICATIONSSPECIFICATIONS
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INTRODUCTIONThis Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either clicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard
ALDEHYDES & KETONESALDEHYDES & KETONESKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
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CONTENTS• Prior knowledge
• Bonding in carbonyl compounds
• Structural differences
• Nomenclature
•Oxidation
• Nucleophilic addition
•2,4-dinitrophenylhydrazine
ALDEHYDES & KETONESALDEHYDES & KETONES
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Before you start it would be helpful to…
• know the functional groups found in organic chemistry
• know the arrangement of bonds around carbon atoms
• recall and explain the polarity of covalent bonds
ALDEHYDES & KETONESALDEHYDES & KETONES
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CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar
PLANAR WITH BOND
ANGLES OF 120°
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CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these
PLANAR WITH BOND
ANGLES OF 120°
P ORBITAL
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CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi () bond
PLANAR WITH BOND
ANGLES OF 120°
P ORBITAL
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CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi () bond
PLANAR WITH BOND
ANGLES OF 120°
P ORBITAL
ORBITAL OVERLAP
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CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi () bond
PLANAR WITH BOND
ANGLES OF 120°
P ORBITAL
ORBITAL OVERLAP
NEW ORBITAL
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CARBONYL COMPOUNDS - BONDINGCARBONYL COMPOUNDS - BONDING
Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi () bond
as oxygen is more electronegative than carbon the bond is polar
PLANAR WITH BOND
ANGLES OF 120°
P ORBITAL
ORBITAL OVERLAP
NEW ORBITAL
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CARBONYL COMPOUNDS - STRUCTURECARBONYL COMPOUNDS - STRUCTURE
Structure carbonyl groups consists ofa carbon-oxygen double bond
the bond is polar due to thedifference in electronegativity
Difference ALDEHYDES - at least one H attached to the carbonyl group
C = OH
CH3
C = OH
H
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CARBONYL COMPOUNDS - STRUCTURECARBONYL COMPOUNDS - STRUCTURE
Structure carbonyl groups consists ofa carbon-oxygen double bond
the bond is polar due to thedifference in electronegativity
Difference ALDEHYDES - at least one H attached to the carbonyl group
KETONES - two carbons attached to the carbonyl group
C = OH
CH3
C = OH
H
C = OCH3
CH3
C = OC2H5
CH3
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CARBONYL COMPOUNDS - FORMULAECARBONYL COMPOUNDS - FORMULAE
Molecular C3H6O
Structural C2H5CHO CH3COCH3
Displayed
Skeletal
C = OH
C2H5
C = OCH3
CH3
H C C C H
H O H
H H
H C C C O
H H H
H H
O O
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CARBONYL COMPOUNDS - NOMENCLATURECARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes C2H5CHO propanal
Ketones CH3COCH3 propanone
CH3CH2COCH3 butanone
CH3COCH2CH2CH3 pentan-2-one
CH3CH2COCH2CH3 pentan-3-one
C6H5COCH3 phenylethanone
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CARBONYL COMPOUNDS - FORMATIONCARBONYL COMPOUNDS - FORMATION
ALDEHYDES Oxidation ofprimary (1°) alcohols RCH2OH + [O] ——> RCHO + H2O
beware of further oxidation RCHO + [O] ——> RCOOH
Reduction ofcarboxylic acids RCOOH + [H] ——> RCHO + H2O
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CARBONYL COMPOUNDS - FORMATIONCARBONYL COMPOUNDS - FORMATION
ALDEHYDES Oxidation ofprimary (1°) alcohols RCH2OH + [O] ——> RCHO + H2O
beware of further oxidation RCHO + [O] ——> RCOOH
Reduction ofcarboxylic acids RCOOH + [H] ——> RCHO + H2O
KETONES
Oxidation ofsecondary (2°) alcohols RCHOHR + [O] ——> RCOR + H2O
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CARBONYL COMPOUNDS - IDENTIFICATIONCARBONYL COMPOUNDS - IDENTIFICATION
Method 1 strong peak around 1400-1600 cm-1 in the infra red spectrum
Method 2 formation of an orange precipitate with 2,4-dinitrophenylhydrazine
Although these methods identify a carbonyl group, they cannot tell the differencebetween an aldehyde or a ketone. To narrow it down you must do a second test.
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i) oxidation with Fehling’s or Benedict’s solution, Tollens’ reagent and acidified
dichromate(VI) ionsii) reduction with lithium
tetrahydridoaluminate (lithium aluminium hydride) in dry ether
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CARBONYL COMPOUNDS - IDENTIFICATIONCARBONYL COMPOUNDS - IDENTIFICATION
Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent
Tollen’sReagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not
ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+
the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s)
the test is known as THE SILVER MIRROR TEST
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CARBONYL COMPOUNDS - IDENTIFICATIONCARBONYL COMPOUNDS - IDENTIFICATION
Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent
Tollen’sReagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not
ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+
the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s)
the test is known as THE SILVER MIRROR TEST
Fehling’sSolution contains a copper(II) complex ion giving a blue solution on warming, it will oxidise aliphatic (but not aromatic) aldehydes the copper(II) is reduced to copper(I)
a red precipitate of copper(I) oxide, Cu2O, is formed
The silver mirror test is the better alternative as it works with all aldehydes
Ketones do not react with Tollen’s Reagent or Fehling’s Solution
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CARBONYL COMPOUNDS - CHEMICAL PROPERTIESCARBONYL COMPOUNDS - CHEMICAL PROPERTIES
OXIDATION
• provides a way of differentiating between aldehydes and ketones• mild oxidising agents are best• aldehydes are easier to oxidise
• powerful oxidising agents oxidise ketones to a mixture of carboxylic acids
ALDEHYDES easily oxidised to acids
RCHO(l) + [O] ——> RCOOH(l)CH3CHO(l) + [O] ——> CH3COOH(l)
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CARBONYL COMPOUNDS - CHEMICAL PROPERTIESCARBONYL COMPOUNDS - CHEMICAL PROPERTIES
OXIDATION
• provides a way of differentiating between aldehydes and ketones• mild oxidising agents are best• aldehydes are easier to oxidise
• powerful oxidising agents oxidise ketones to a mixture of carboxylic acids
ALDEHYDES easily oxidised to acids
RCHO(l) + [O] ——> RCOOH(l)CH3CHO(l) + [O] ——> CH3COOH(l)
KETONES oxidised under vigorous conditions to acids with fewer carbons
C2H5COCH2CH3(l) + 3 [O] ——> C2H5COOH(l) + CH3COOH(l)
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CARBONYL COMPOUNDS - CHEMICAL PROPERTIESCARBONYL COMPOUNDS - CHEMICAL PROPERTIES
ReductionReduction
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CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism occurs with both aldehydes and ketones
involves addition to the C=O double bond
unlike alkenes, they are attacked by nucleophiles
attack is at the positive carbon centre due to thedifference in electronegativities
alkenes are non-polar and are attacked by electrophiles
undergoing electrophilic addition
C=C ELECTROPHILESALKENESCARBONYLS
NON-POLARC=O POLAR NUCLEOPHILES
ADDITIONADDITION
Bond Attacking speciesGroup Polarity Result
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iii) nucleophilic addition of HCN in the presence of KCN, using curly
arrows, relevant lone pairs, dipoles and evidence of optical activity to
show the mechanism
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CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive COne of the C=O bonds breaks; a pair of electrons goes onto the O
STEP 1
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CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 Lone pair on nitrogen acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2 A pair of electrons is used to form a bond with H+
STEP 2STEP 1
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CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
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iv) the reaction with 2.4-dinitrophenylhydrazine and its use to detect the presence of a carbonyl
group and to identify a carbonyl compound given data of the
melting temperatures of derivatives
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2,4-DINITROPHENYLHYDRAZINE2,4-DINITROPHENYLHYDRAZINE
Structure
Use reacts with carbonyl compounds (aldehydes and ketones)used as a simple test for aldehydes and ketonesmakes orange crystalline derivatives - 2,4-dinitrophenylhydrazonesderivatives have sharp, well-defined melting pointsalso used to characterise (identify) carbonyl compounds.
Identification / characterisationA simple way of characterising a compound (finding out what it is) is to
measure the melting point of a solid or the boiling point of a liquid.
C6H3(NO2)2NHNH2
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2,4-DINITROPHENYLHYDRAZINE C2,4-DINITROPHENYLHYDRAZINE C66HH33(NO(NO22))22NHNHNHNH22
The following structural isomers have similar boiling points because of similarvan der Waals forces and dipole-dipole interactions. They would be impossibleto identify with any precision using boiling point determination.
CHO CHO CHOCl
ClCl
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2,4-DINITROPHENYLHYDRAZINE C2,4-DINITROPHENYLHYDRAZINE C66HH33(NO(NO22))22NHNHNHNH22
The following structural isomers have similar boiling points because of similarvan der Waals forces and dipole-dipole interactions. They would be impossibleto identify with any precision using boiling point determination.
Boiling point 213°C 214°C 214°C
CHO CHO CHOCl
ClCl
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2,4-DINITROPHENYLHYDRAZINE C2,4-DINITROPHENYLHYDRAZINE C66HH33(NO(NO22))22NHNHNHNH22
The following structural isomers have similar boiling points because of similarvan der Waals forces and dipole-dipole interactions. They would be impossibleto identify with any precision using boiling point determination.
Boiling point 213°C 214°C 214°C
Melting point of2,4-dnph derivative 209°C 248°C 265°C
By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point,it will be easier to identify the unknown original carbonyl compound.
CHO CHO CHOCl
ClCl
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2,4-DINITROPHENYLHYDRAZINE C2,4-DINITROPHENYLHYDRAZINE C66HH33(NO(NO22))22NHNHNHNH22
The following structural isomers have similar boiling points because of similarvan der Waals forces and dipole-dipole interactions. They would be impossibleto identify with any precision using boiling point determination.
Boiling point 213°C 214°C 214°C
Melting point of2,4-dnph derivative 209°C 248°C 265°C
By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point,it will be easier to identify the unknown original carbonyl compound.
CHO CHO CHOCl
ClCl
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v) iodine in the presence of alkali (4.8.2c)
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Iodoform reaction
A positive result - the pale yellow precipitate of triiodomethane (iodoform) - is given by an aldehyde or
ketone containing the grouping:
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THE CHEMISTRYTHE CHEMISTRYOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES
THE ENDTHE END
©©2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING