Organic II Flashcards
What are optical isomers?
Optical isomers are molecules which contain a chiral carbon (a carbon atom bonded to four different substituents), which are non-superimposable mirror images of each other and rotate the plane of plane-polarised light by equal amounts in opposite directions.
What is a racemic mixture?
A 50/50 mixture of the two enantiomers of a compounds, such that the mixture has no optical activity. The rotation of plane-polarised light in one direction by one enantiomer is cancelled out by the rotation of the other enantiomer in the other direction.
Does SN1 produce a racemic mixture (when a chiral compound is formed)?
Yes as during SN1, a carbocation intermediate is formed, which has a trigonal planar structure. There is an equal chance of the nucleophile attacking from above or below the plane of the molecule, and as such, an equal mixture (racemate) of the two enantiomers is formed.
Does SN2 produce a racemic mixture (when a chiral product is made)?
No as the nucleophile will always attack the molecule from the opposite side to the leaving group, so only one enantiomer is formed. Therefore, the product is optically active.
What is the carbonyl functional group?
C=O
What is the aldehyde functional group?
R-CHO
What is the ketone functional group?
R-CO-R’
How do you name aldehydes and ketones?
Ketones end in -one
Aldehydes end in -al
The aldehyde and ketone functional groups take priority over most other A Level functional groups (e.g. is a compound had alcohol and ketone groups, the ketone would take the ending (e.g. 3-hydroxybutanone)).
How do the boiling points of aldehydes and ketones compare to alcohols?
Aldehydes and ketones have lower boiling points than similar alcohols because they cannot form hydrogen bonds between them (but alcohols can). However, they do still have pd-pd forces and London forces, so have higher boiling points than equivalent alkanes.
Are aldehydes and ketones soluble in water?
They have two lone pairs on the double bonded oxygen, so they can accept hydrogen bonds from the delta + hydrogens of water (but they can’t form hydrogen bonds themselves as they don’t have a positive enough H atom (not a strong enough dipole)). Smaller aldehydes and ketones are soluble in water because of this hydrogen bonding, but as the molecules get larger, they become less soluble because the large non-polar alkyl chains dominate and the molecule will not have enough water around it to be fully dissolved. This is because the hydrogen bonds between water molecules and London forces between aldehyde/ketone molecules are stronger than the hydrogen bonds which could form between the aldehyde/ketone and water.
What is Tollen’s reagent and how does it work?
Tollen’s reagent is a colourless solution of silver nitrate dissolved in aqueous ammonia. It can be heated in a test tube in the test for aldehydes, a silver mirror is the positive result. This is because Tollen’s is a mild oxidising agent, which oxidises the aldehyde to a carboxylic acid, but won’t oxidise a ketone. The silver ions in the solution are reduced to silver atoms, which deposit on the surface of the test tube, forming the silver mirror.
What do Benedict’s and Fehling’s solution do?
They both form a brick red precipitate from a blue solution in the presence of an aldehyde (because they are mild oxidising agents). Cu2+ (blue) are reduced to Cu1+ (red), forming the Cu2O precipitate. This will not happen with a ketone as they are not readily oxidised.
How can acidified potassium dichromate (VI) be used in carbonyl testing?
Acidified potassium dichromate (VI), is an oxidising agent, which will oxidise aldehydes to carboxylic acids, but won’t oxidise ketones. The solution will turn from orange (Cr6+) to green (Cr3+).
How can aldehydes and ketones be reduced?
Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using a reducing agent. You could use LiAlH4 (lithium tetrahydroaluminate (III)/lithium aluminium hydride) in dry ether (as it is a strong reducing agent and reacts violently with water) or NaBH4 in dry ether (not as strong a reducing agent).
By which mechanism does hydrogen cyanide react with carbonyls?
Nucleophilic addition.
Describe the reaction mechanism between hydrogen cyanide and a carbonyl.
Cyanide ions (from KCN) are in solution. The carbon has a lone pair and a negative charge, so the CN ions act as a nucleophile. The C of the carbonyl group has a delta positive charge because oxygen is more electronegative than carbon, so the cyanide ion attacks this C. This breaks one of the bonds (pi bond) between oxygen and carbon. This oxygen has a lone pair and a negative charge. The lone pair attacks the delta positive hydrogen of HCN, forming an O-H bond and breaking the H-CN bond. This forms a hydroxynitrile and regenerates the cyanide ion (which acts as a catalyst).
