CARBONYL COMPOUNDS - Aldehydes and Ketones Structure
•carbonyl groups consists of a carbon-oxygen double bond •the bond is polar  due to the difference in electronegativity •aldehydes / ketones differ in what is attached to the carbon ALDEHYDES -  at least one H attached to the carbonyl group HCHO      or CH 3CHO C 6H 5CHO KETONES    -  two carbons attached to the carbonyl group CH 3COCH 3C 2H 5COCH 3C 6H 5COCH 3Bonding
•the carbonyl carbon is sp 2 hybridised and three sigma (σ) 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 (π) bond •as oxygen is more electronegative than carbon the bond is polar Naming
•aldehydes AL •ketones ONE •pick the longest chain of carbon atoms which includes the C=O •substituent positions are based on the carbon with the O attached CH 3CH 2CH 2CH 2CH 2CHO CH 3COCH 2CH 2CH 2CH 3  hexanal hexan-2-one C O O C O C Q.1
Draw structures for, and name, all the carbonyl compounds with molecular formulae;
a)  C 4H 8O b)  C 5H 10O c)  C 6H 12O
C=O C C C=O H C C=O H H
Formation of carbonyl compounds from alcohols
Aldehydes •  Oxidation of primary (1°) alcohols  -  risk of oxidation to acids
eg CH 3CH 2OH (l)  +  [O]    —>    CH 3CHO (l)  +  H 2O (l)
ethanol      ethanal
however, this can happen CH 3CHO (l)  +  [O]    —>    CH 3COOH (l)
ethanal ethanoic acid
• it is essential to distil off the aldehyde  before it gets oxidised to the acid
• the alcohol is dripped into a warm solution of acidified K 2Cr 2O 7
• the aldehyde has a low boiling point  - no hydrogen bonding - it distils off
• if it didn’t distil off it would be oxidised to the equivalent carboxylic acid
• to oxidise an alcohol straight to the acid you would reflux the mixture
Ketones •  Oxidation of secondary (2°) alcohols.
eg CH 3CHOHCH 3(l)  +  [O]    —>    CH 3COCH 3(l)  +  H 2O (l)
propan-2-ol    propanone
DISTILLATION gives an ALDEHYDE REFLUXING
gives a
CARBOXYLIC ACID
Q.2Which alcohol would you use to make the following?
•  C 2H 5CHO
•  C 2H 5COCH 3
•  hexanal
• 3-methylhexan-2-one
• 3-methylpentanal
CHEMICAL PROPERTIES OF CARBONYL COMPOUNDS
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 carboxylic acid mixtures
ALDEHYDES easily oxidised to acids  e.g. RCHO(l)  +  [O]  ——>  RCOOH(l)
CH3CHO(l) +  [O]  ——>  CH3COOH(l) KETONES only oxidised under vigorous conditions to acids with fewer carbons.
<  C2H5COCH2CH3(l)  +  3 [O]  ——>  C2H5COOH(l)+  CH3COOH(l)
Q.3What product (if any) is formed when the following undergo mild oxidation?
•  C2H5CHO
•  C2H5COCH3
•  hexanal
•  3-methylhexan-2-one
•  3-methylpentanal
•  cyclohexanone
IDENTIFYING A CARBONYL COMPOUND
Methods•  characteristically strong peak at 1400-1600 cm-1in the infra red spectrum    or •  formation of orange crystalline precipitate with 2,4-dinitrophenylhydrazine
BUT to narrow it down to an aldehyde or ketone you must do a second test Differentiation•  to distinguish an aldehyde from a ketone you need a mild oxidising agent ...
Tollens’ Reagent•  ammoniacal silver nitrate
•  contains the diammine silver(I) ion -  [Ag(NH3)2 ]+
•  acts as a mild oxidising agent and will oxidise aldehydes but not ketones
•  the silver(I) ion is reduced to silver Ag+(aq)  +  e¯  —>  Ag(s)
•  the test is known as THE SILVER MIRROR TEST
Fehling’s Solution•  contains copper(II) ions complexed with tartrate ions
•  on warming, it will oxidise aliphatic (but not aromatic) aldehydes
•  copper(II) is reduced to a  red precipitate of copper(I) oxide, Cu2O
The silver mirror test is the better alternative as it works with all aldehydes.
Ketones do not react with Tollens’ Reagent or Fehling’s Solution.
Q.4Which of the following produce an orange precipitate with 2,4-dinitrophenylhydrazine?
•  C2H5OH•  3-methylhexan-2-one
•  C2H5COCH3•  cyclohexanol
•  hexanal•  3-methylpentan-1-ol
Q.5Which of the following produce a silver mirror with Tollens’ reagent?
•  C2H5CHO•  3-methylhexan-2-one
•  C2H5COCH3•  cyclohexanone
•  hexanal•  3-methylpentanal
NUCLEOPHILIC ADDITION REACTIONS
Mechanism •occurs with both aldehydes and ketones
characterise•involves addition to the polar C=O double bond
•attack is by nucleophiles at the positive carbon centre •alkenes are non-polar and are attacked by electrophiles
REDUCTION Reagent  sodium tetrahydridoborate(III) (sodium borohydride), NaBH 4
Conditions  aqueous or alcoholic solution
Mechanism  Nucleophilic addition  (also reduction as it is addition of H¯)Nucleophile
H¯ (hydride ion)Product(s)  Aldehydes
REDUCED to primary (1°) alcohols Ketones REDUCED to secondary (2°) alcohols
Equation(s) CH 3CHO    +    2[H]      ——>    CH 3CH 2OH
CH 3COCH 3  +  2[H]    ——>    CH 3CHOHCH 3
Step 1H¯ is a nucleophile and attacks the C δ+
An electron pair from the C=C moves onto O making it -ive
Step 2
A lone pair on oxygen removes a proton from water Overall, there is addition of hydrogen (reduction)Bond
Polarity Attacked by Result Carbonyl
C=O Polar Nucleophiles Addition Alkene
C=C Non-polar Electrophiles Addition C
H
O CH 3
OH H C CH 3H C CH 3H O H HO
H O H +H Q.6Draw a diagram to indicate
the bonding in NaBH 4.

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