aldehydes and ketones

molecules that contain the C=O functional group at a level we encounter them as ketones, aldehydes, carboxylic acids and esters

strongly polar as O is more electronegative than C so positive C atom attracts nucleophiles

a nucleophile is an electron pair donor, and it is addition because we break the double bond in C=O and add a species in rather than swapping out

with the cyanide ion- remember it cannot come from nothing, it usually comes from KCN or HCN

hydrogen cyanide is a toxic gas that is difficult to contain so these reactions are usually done with potassium or sodium cyanide, in water followed by a dilute acid

if we use potassium or sodium cyanide, we need protons to attach to the negatively charged O- that forms as an intermediate BUT if you use HCN, the H+ comes from the HCN

a hydroxynitrile- very useful in making further products

for an asymmetric planar carbonyl group, the nucleophile can attack from above or below the plane equally this produces two optical isomers in racemic mixture when allowed to react naturally

pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces

smaller ones are soluble in water because they can form hydrogen bonds with water

aldehydes to carboxylic acid reagent; potassium dichromate solution and dilute sulfuric acid conditions; heat under reflux

RCHO + [O] ——> RCO2H

orange dichromate ion (Cr2O72-) reduces to green Cr3+ ion

Tollens reagent formed by mixing aqueous ammonia and silver nitrate heat gently aldehydes are oxidised by Tollens’ reagent into a carboxylic acid (silver ions reduced to silver atoms) silver mirror forms

Fehling’s solution contains blue Cu2+ ions conditions; heat gently reaction; aldehydes are oxidised by Fehling’s solution to carboxylic acid (copper ions are reduced to copper oxide) observation; blue Cu2+ ions in solution change to brick red precipitate of Cu2O

aldehydes can be oxidised to carboxylic acids but ketones cannot

reagents; NaBH4 in aqueous ethanol conditions; room temp and pressure reducing agents such as NaBH4 (sodium tetrahydridoborate) and LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols

aldehydes reduced to primary alcohols ketones reduced to secondary alcohols IT IS A NUCLEOPHILIC ADDITION MECHANISM

it contains a source of nucleophilic hydride ions (H-) which are attracted to the positive carbon in the C=O bond

using [H] in the equation eg CH3COCH3 + 2[H] —–> CH3CH(OH)CH3

reduction of aldehydes leads to primary alcohols that are not chiral so not optically active reduction of symmetrical ketones also leads to optically inactive substances

catalytic hydrogenation reagent; hydrogen and nickel catalyst conditions; high pressure example equation; CH3CHO + H2 —-> CH3CH2OH

there will be a higher concentration of the CN- ion as these compounds will completely ionise HCN is a weak acid and will only partially ionise

toxic/ poisonous/ corrosive