Download Carbonyl Compounds_ Properties and Reactions

Reactions of carbonyl compounds
L.O. To confidently identify different carbonyl compounds.
To find out how carbonyl compounds react.
H
O
H
O
H3C
H
O
O
O
Uses of carbonyl compounds
1.
Propanone: Industrial solvent in paints, varnishes, nail-polish
removers.
2. Methanal:
a. Preserving and embalming, as a germicide and insecticide
b. Manufacturing plastic coatings such as Bakelite and melamine
c. Manufacturing polymer adhesives (e.g. wood glues)
3. Flavourings: Benzaldehyde (C6H5CHO) – fresh almonds
Heptan-2-one (C5H11COCH3) – blue cheese!
Carbonyl compounds - bonding
The carbon is sp2 hybridised and three sigma (s) bonds are planar.
The unhybridised 2p orbital of carbon is at 90° to these.
This overlaps with a 2p orbital of oxygen to form a pi (p) bond
P ORBITAL
PLANAR
WITH
BOND
ANGLES
OF 120°
ORBITAL
OVERLAP
NEW
ORBITAL
as oxygen is more electronegative than carbon the bond is polar
Physical properties of carbonyl compounds
Polarity is LESS than that of hydroxyl group in alcohols.
Therefore, aldehydes and ketones have weaker dipole-dipole
interactions and lower b.p. than alcohols of comparable Mr (hence
good in paints and varnishes as evaporate easily).
Carbonyls show a limited/lack of hydrogen bonding between
molecules, whereas the corresponding alcohol will show extensive
intermolecular H bonding.
Weaker polarity means aldehydes and ketones mix well with polar
solvents such as water and will dissolve many organic compounds.
Identification reactions of carbonyl compounds
2,4-dinitrophenylhydrazine
CONDENSATIO
N REACTION
C6H3(NO2)2NHNH2
-Carbonyl group is detected using Brady’s reagent
(solution of 2,4-dinitrophenylhydrazine [2,4-DNPH] in dilute acid)
-Simple test for aldehydes and ketones
-Bright orange-yellow crystals (ppt) formed which identifies
presence of carbonyl group (C=O).
-Products are derivatives: 2,4-dinitrophenylhydrazones. These
have sharp, well-defined melting points so can be used to
characterise (identify) carbonyl compounds.
Identification reactions of carbonyl compounds
Distinguishing aldehydes from ketones: - use a mild oxidising agent
In each test the ALDEHYDE REDUCES the reagents used,
producing a VISIBLE COLOUR CHANGE.
1. Tollen’s Reagent
ammoniacal silver nitrate
Mild oxidising agent which will oxidise aldehydes but not ketones
Procedure: Warm aldehyde or ketone with Tollen’s reagent.
Contains the diammine silver(I) ion -
[Ag(NH3)2 ]+
The silver(I) ion is reduced to silver
Ag+(aq) + eThe test is known as THE SILVER MIRROR TEST
Ag(s)
Identification reactions of carbonyl compounds
2. Fehling’s Solution (or Benedict’s solution)
Procedure: Warm aldehyde or ketone with Fehling’s solution.
Contains a copper(II) complex (Cu2+ ions dissolved in aqueous alkali)
ion giving a blue solution on warming.
It will oxidise aliphatic (but not aromatic) aldehydes
2Cu2+ (aq) + 2e- + 2OH- (aq)
Cu2O (s) + H2O (l)
N.B: The silver mirror test is best as it works with ALL aldehydes
Ketones do not react with Tollen’s Reagent or Fehling’s Solution
Identification reactions of carbonyl compounds
3. Acidified dichromate (VI) Ions
Procedure: Warm aldehyde or ketone with acidified potassium
dichromate (VI).
Dilute H2SO4 used as source of H+ ions.
K2Cr2O7 used as source of Cr2O72Aldehydes will be oxidised BUT show no reaction with ketones
Cr2O72- (aq) + 14H+ (aq) + 6e-
ORANGE (CrVI)
2Cr3+ (aq) + 7H2O (l)
GREEN (CrIII)
Chemical properties of carbonyl compounds
Oxidation
•
•
•
•
Used to differentiate between aldehydes and ketones.
Mild oxidising agents work best (Tollen’s reagent, Fehling’s etc).
Aldehydes are easier to oxidise.
Powerful oxidising agents will oxidise ketones. Ketones oxidise
to a mixture of carboxylic acids.
ALDEHYDES: Easily oxidised to the corresponding carboxylic acid
RCHO(l)
CH3CHO(l)
+
[O]
+
[O]
RCOOH(l)
CH3COOH(l)
KETONES: Oxidised under vigorous conditions to carboxylic acids with fewer
carbon atoms (MCPBA [oxidises to ester], or conc KMn04 +reflux)
C2H5COCH2CH3(l)
+ 3 [O]
C2H5COOH(l)
+
CH3COOH(l)
Nucleophilic addition reactions
Both aldehydes and ketones undergo Nu- addition reactions.
Reaction involves addition to the C=O double bond.
For carbonyls, attack is at the positive carbon centre.
This is due to the difference in electronegativity between C and O.
N.B: This is UNLIKE ALKENES, which are attacked by E+.
This is because they are NON-POLAR
Group
Bond
Polarity
Attacking species
Result
ALKENES
C=C
NON-POLAR
ELECTROPHILES
ADDITION
CARBONYLS
C=O
POLAR
NUCLEOPHILES
ADDITION
Nucleophilic addition reactions
Reagent: Hydrogen cyanide - HCN (in the presence of KCN +
small amount of dilute H2SO4).
KCN is needed to help generate the Nu- (cyanide nucleophile CN-).
Acid used as source of H+ ions.
Conditions: Room temperature
Product(s): Hydroxynitrile (cyanohydrin)
Equation: CH3CHO
N.B:
+
HCN
CH3CH(OH)CN
2-hydroxypropanenitrile
HCN is a weak acid and has difficulty dissociating into ions
HCN
H+
+
CN¯
Therefore the reaction is aided by KCN and dilute acid which help
produce more of the nucleophilic CN¯
Nucleophilic addition reactions
Mechanism: Nucleophilic addition
STEP 1
STEP 2
Step 1: CN- acts as nucleophile, attacking the slightly positive carbon.
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+.
Overall, HCN has added to the carbonyl compound.
Optical activity and Nucleophilic addition
Mechanism: Nucleophilic addition
STEP 1
STEP 2
Optical activity: The mechanism depends on both the concentration of
the nucleophile and the concentration of the carbonyl (it is
BIMOLECULAR).
The product that results contains a chiral centre (carbonyl carbon) and
so will show optical activity.
N.B. See nucleophilic substitution reactions of halogenoalkanes for
other examples of stereospecific mechanisms (resulting in retention
or loss of optical isomerism)
Alternative reagents for HCN reaction
Reagent: Hydrogen cyanide - HCN (in the presence of trace
amounts of base [NaOH]).
NaOH helps generate CN- ions as HCN only very weakly acidic
Conditions: Room temperature
The reaction proceeds as previously described.
Reduction of carbonyl compounds
Possible reagents:
1. Sodium tetrahydridoborate (III) (sodium borohydride),NaBH4 (aq)
2. Lithium tetrahydridoaluminate (III), LiAlH4 in ether, followed by
water.
N.B: The above 2 are the most common to be used with carbonyl
compounds.
3. Sodium in ethanol (ethanol generates hydrogen)
4. Hydrogen under pressure with a nickel, platinum or palladium
catalyst
Reduction of carbonyl compounds with NaBH4
Reagent: Sodium tetrahydridoborate (III) (sodium borohydride),NaBH4
Conditions: Aqueous solution at room temperature
Mechanism: Nucleophilic addition (also reduction as it is addition of H)
Nucleophile:
H- (hydride ion)
Product(s):
Alcohols
Aldehydes are REDUCED to primary (1°) alcohols.
Ketones are REDUCED to secondary (2°) alcohols.
Equation(s):
CH3CHO
+
CH3COCH3 +
2[H]
2[H]
CH3CH2OH
CH3CHOHCH3
N.B: The water provides a proton to make the alcohol AND
NaBH4 does not reduce C=C
Reduction of carbonyl compounds with LiAlH4
Reagent: Lithium tetrahydridoaluminate (lithium aluminium hydride)
LiAlH4
Conditions: Dry ether followed by hydrolysis by water
Mechanism: Nucleophilic addition (also reduction as it is addition of H)
Nucleophile:
AlH4- (dry conditions needed to prevent hydrolysis)
Product(s):
Alcohols
Aldehydes are REDUCED to primary (1°) alcohols.
Ketones are REDUCED to secondary (2°) alcohols.
Can you draw the mechanism for this reaction?
Reduction of carbonyl compounds with LiAlH4
H
R
C
O
H Al
H
H
H
H
O H
O
C H
H
R
H
OH
C H
R
OH
Reduction of carbonyl compounds with other
functionalities
Functional groups containing multiple bonds can be reduced
C=C
C=O
CN
is reduced to
is reduced to
is reduced to
Hydrogen:
H2
H- (nucleophile)
Reactions:
Hydrogen reduces C=C and C=O bonds
CH2 = CHCHO
+
CH-CH
CH-OH
CH-NH2
4[H]
CH3CH2CH2OH
Hydride ion H- reduces C=O bonds ONLY
CH2 = CHCHO
+
2[H]
CH2=CHCH2OH
Reduction of carbonyl compounds with other
functionalities
EXPLANATION
C=O is polar so is attacked by the nucleophilic HC=C is non-polar so is not attacked by the nucleophilic HKey point when choosing reducing agent – which group do you want
reduced?
Reduction of carbonyl compounds with other
functionalities
What are the products when Compound X is reduced?
COMPOUND X
H2
NaBH4
Reduction of carbonyl compounds with other
functionalities
What are the products when Compound X is reduced?
COMPOUND X
H2
NaBH4
C=O is polar so is attacked by the nucleophilic HC=C is non-polar so is not attacked by the nucleophilic H-
Iodoform reaction – identification test for
carbonyl compounds containing a specific group
Aldehydes and ketones containing the CH3C=O group will give a
positive result with the triiodomethane reaction.
Therefore, only aldehyde to give a positive result is ETHANAL
Alcohols will also give a positive result if they can first oxidise
to a ketone (secondary alcohols) containing the above group.
Reaction: The organic compound added to a mixture of IODINE and
dilute NaOH (or a mixture of KI and NaOCl [sodiun chlorate])
Result: A pale yellow precipitate of iodoform (CHI3)
Iodoform reaction – a short practical to see
it in action…..
1. Add 4cm3 of NaOH (aq) to each of 3 test tubes.
2. To the first test tube add 6 drops of compound A.
3. To the second test tube add 6 drops of compound B.
4. To the third test tube ass 6 drops of compound C.
5. Add I2 (aq) dropwise to each test tube until a faint brown colour
remains.
6. Allow the tubes to stand for a short time and then record your
observations.
7. Which of the compounds has produced the CHI3 ppt and so which
contain(s) the CH3C=O group?