Structures of Aldehydes and Ketones• Both aldehydes and ketones contain a carbonyl group• Aldehydes have at least one H attached, while ketones have

two C’s attached to the carbonyl• A carbonyl consists of a C double-bonded to an O• Like in an alkene, the double bond consists of one sigma and

one pi bond• The carbonyl is a very polar group

- O is more electronegative than C, so C-O bonds are polar- Also, the carbonyl has two resonance forms- This polarity makes carbonyls chemically reactive

H

O O O OO

δ+δ-

Aldehyde Ketone

Naming Ketones• Parent name ends in -one• Find longest chain containing the carbonyl group• Number C’s starting at end nearest carbonyl group• Locate and number substituents and give full name

- use a number to indicate position of carbonyl group- cyclic ketones have cyclo- before the parent name; numbering begins at the carbonyl group, going in direction that gives substituents lowest possible numbers- use a prefix (di-, tri-) to indicate multiple carbonyl groups in a compound

O O O

propanone(acetone)

butanone 2-pentanone

O

Br

4-bromo-3-methylcyclohexanone

Naming Aldehydes• Parent name ends in -al• Find longest chain containing the carbonyl group• Number C’s starting at end nearest carbonyl group• Locate and number substituents and give full name

- aldehydes take precedence over ketones and alcohols in naming- ketones are called oxo as a secondary group- alcohols are called hydroxy as a secondary group- the smallest aldehydes are usually named with common names- we will not name cyclic aldehydes (except benzaldehyde)

H H

O

H

O

H

O

H

OOOH

methanal(formaldehyde)

ethanal(acetaldehyde)

3-methylbutanal 5-hydroxy-3-oxohexanal

Physical Properties of Aldehydes and Ketones

• Because the carbonyl group is polar, aldehydes and ketones have higher boiling points than hydrocarbons

• However, they have no H attached to the O, so do not have hydrogen bonding, and have lower boiling points than alcohols

• Like ethers, aldehydes and ketones can hydrogen bond with water, so those with less than 5 carbons are generally soluble in water

• Aldehydes and ketones can be flammable and/or toxic, though generally not highly so

• They usually have strong odors, and are often used as flavorings or scents

Oxidation of Aldehydes• Recall that aldehydes and ketones are formed by the oxidation of

primary and secondary alcohols, respectively• Also recall that aldehydes are readily oxidized to carboxylic

acids, but ketones are not• Tollens’ reagent (silver nitrate plus ammonia) can be used to

distinguish between ketones and aldehydes- with aldehydes the Ag2+ is reduced to elemental silver, which forms a mirror-like coat on the reaction container

• Sugars (like glucose) often contain a hydroxy group adjacent to an aldehyde

- Benedict’s reagent (CuSO4) can be used to test for this type of aldehyde; the blue Cu2+ forms Cu2O, a red solid

Reduction of Aldehydes and Ketones• Reduction can be defined as a loss in bonds to O or a gain in bonds

to H• Aldehydes and ketones can be reduced to form alcohols

- Aldehydes form primary alcohols- ketones form secondary alcohols

• Many different reducing agents can be used, including H2, LiAlH4 (lithium aluminum hydride) and NaBH4 (sodium borohydride)

• However, NaBH4 is usually the reagent of choice- hydrogenation will also reduce alkenes and alkynes if present

- LiAlH4 is more reactive than NaBH4, but reacts violently with water and explodes when heated above 120º C

H

ONaBH4

OH

ONaBH4

OH

Addition of Water to Aldehydes and Ketones• H2O can add across the carbonyl of an aldehyde or a ketone,

similar to the addition of H2O to an alkene• A partial positive H from water bonds to the partial negative

carbonyl O, and the partial negative O from water bonds to the partial positive carbonyl C

• The product of this reversible reaction is a hydrate (a 1,1-diol)• In general, the equilibrium favors the carbonyl compound, but

for some small aldehydes the hydrate is favored• The reaction can be catalyzed by either acid or base

H

O H3O+

H

HO OH

0.1% 99.9%

O H3O+ HO OH

0.1%99.9%

Mechanism of Acid-Catalyzed Hydration of Formaldehyde• First, the carbonyl O is protonated by the acid catalyst• Next, H2O attacks the carbonyl carbon to form a protonated

hydrate• Finally, H2O removes the proton to form the hydrate

+

HO

H

H

H

O

H

OH

+H

OH

H

OH

+ HO

H

H

HO O

H

H

H

HO O

H

H +H

OH

H

HO OH

+

HO

H

H

Addition of Alcohols to Aldehydes and Ketones• Alcohols can add to aldehydes and ketones using an acid catalyst• Addition of 2 alcohols produces an acetal (a diether)• The reaction intermediate, after addition of one alcohol, is a

hemiacetal (not usually isolated)• This is a reversible reaction

- removal of H2O favors acetal

- addition of H2O favors aldehyde or ketone• Acetals are often used as protecting groups in organic synthesis

O

+ CH3OHAcid

Cat.

HO OCH3

+ CH3OHAcid

Cat.

H3CO OCH3

Hemiacetal Acetal

Formation of Cyclic Hemiacetals• When an aldehyde or a ketone is in the same molecule as an

alcohol, a cyclic hemiacetal can form• These are more stable than the non-cyclic ones and can be

isolated• Sugars, like glucose and fructose, exist primarily in the cyclic

hemiacetal form• When an alcohol adds to a cyclic hemiacetal, a cyclic acetal is

formed (this is how sugars bond together in polysaccharides)

O

H

OH Acid

Cat.O

OHH

Stereoisomers• Recall that constitutional isomers have the same molecular

formula, but the atoms are bonded in a different order

• Stereoisomers have the same molecular formula, and the same bonding order, but the atoms are arranged differently in 3-D space

• There are two types of stereoisomers:

- enantiomers are non-superimposable mirror images

- diasteriomers are stereoisomers that are not mirror images (cis-trans isomers a type of diastereomers)

OH

HBr

CH3

OH

HBr

H3C

Enantiomers

CH3 CH3

H H

H CH3

CH3 H

Diastereomers

Chirality• An object, or a molecule, is chiral if it has a mirror image

that is not superimposable

• The most familiar chiral objects are your hands

- the right hand is the mirror image of the left hand

- no matter how you turn them, they can’t be superimposed

• Many organic compounds are also chiral

- most biomolecules (amino acids, sugars, etc.) are chiral and usually only one of the stereoisomers is used

• In order for a carbon in an organic compound to be chiral, it must have 4 different groups attached (otherwise the mirror image will be superimposable)

Fischer Projections• Fischer projections are a simple way to represent chiral

molecules (especially sugars)

• The bonds to a chiral carbon are shown as crossed perpendicular lines, with the chiral C at the center

- Horizontal bonds are coming towards you (like wedges)

- Vertical bonds are going away from you (like dashes)

• The D and L classification of sugars is based on the simplest sugar, glyceraldehyde

• Compounds with more than one chiral carbon, such as larger sugars, can also be represented as Fischer projections

- Each place where a horizontal line crosses the vertical line represents a carbon