Aldehydes and Ketones

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

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

IUPAC nomenclature of ketones Common system of ketones:

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)

Naming Aldehydes

Nomenclature because the carbonyl group of an aldehyde can only be at the end of a parent chain and numbering must start with it as carbon-1, there is no need to use a number to locate the aldehyde group for unsaturated aldehydes, indicate the presence of a carbon-carbon double bond and an aldehyde by changing the ending of the parent alkane from -ane to -enal; show the location of the carbon-carbon double bond by the number of its first carbon

Nomenclature the IUPAC system retains common names for some aldehydes, including these three

2) Common name

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

Preparation of aldehydes & ketones: From acid chloride: This method is called Rosemund reduction.

(2) From geminal dihalide: Question: convert toluene to benzaldehyde Answer

(3) Partial decarboxylation of salt of acids: Question: Show how could you prepare the following To prepare by above method, we use two molecules

(4) From nitrile: a- Aldehyde: b- Ketone

(5) Oxidation of alcohols: a- Aldehyde: From oxidation of primary alcohol. b- Ketone: From oxidation of secondary alcohol.

(6) Ozonolysis of alkene : H (7) Hydration of alkyne : Only other alkyne except acetylene will give ketone

Synthesis of aromatic ketones via Friedel-Crafts acylation: E.g. Acetophenone Form benzophenone

Synthesis of benzaldehyde Gattermann-Koch aldehyde synthesis b) Gattermann aldehyde synthesis

Chemical reactions of aldehydes & Ketones A-Type I of reaction (Addition reaction) Examples of this addition is addition of HCN, H2, RMgX, HOH, NaSO3H

Formation of hemiacetals and acetals 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

B-Type II of reaction [addition reaction followed by loss of H2O] e.g. (Condensation with amines) Examples:

2- Cannizaro reaction: - It occurs between two aldehydes with no α-hydrogen in presence of base to give an alc. and an acid. The adduct of an aldehyde and OH can transfer hydride ion to another aldehyde C=O resulting in a simultaneous oxidation and reduction (disproportionation)

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

Oxidation Aldehydes are oxidized to carboxylic acids by a variety of oxidizing agents, including potassium dichromate liquid aldehydes are so sensitive to oxidation by O2 of the air that they must be protected from contact with air during storage

Oxidation Ketones resist oxidation by most oxidizing agents, including potassium dichromate and molecular oxygen Tollens’ reagent is specific for the oxidation of aldehydes; if done properly, silver deposits on the walls of the container as a silver mirror

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

Reduction The carbonyl group of an aldehyde or ketone is reduced to an -CHOH group by hydrogen in the presence of a transition-metal catalyst reduction of an aldehyde gives a primary alcohol reduction a ketone gives a secondary alcohol

Reduction reduction by NaBH4 does not affect a carbon-carbon double bond

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

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

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

2- Reduction to hydrocarbon: For examples;

a- Clemmensen reduction: b- Walf-Kishner reduction:

Keto-Enol Tautomerism A carbon atom adjacent to a carbonyl group is called an a-carbon, and a hydrogen atom bonded to it is called an a-hydrogen

Keto-Enol Tautomerism A carbonyl compound that has a hydrogen on an a-carbon is in equilibrium with a constitutional isomer called an enol the name “enol” is derived from the IUPAC designation of it as both an alkene (-en-) and an alcohol (-ol) in a keto-enol equilibrium, the keto form generally predominates