Organic chemistry-carbonyls

what is sterioisomerism?

Molecules with the same structural formula, but a different 3-D arrangement/arrangement in space

what are the two types of sterioisomers?

• E/Z or cis/trans

• Optical isomerism: occurs due to the presence of a chiral carbon. Object and its mirror image.

what are optical isomers called?

• Enontiomers - are is non-superimposable

• A mixture containing an equimolar amount of the two isomers is called a racemix mixture

what are optical isomers/enontiomers

• these are non-superimposable mirror image of each other. The central atom is a chiral centre/carbon

what is a chiral

• A chiral carbon has four different atoms, or groups of atoms bonded to it

• Often referred to as asymmetric carbons

• Both the bonds and the atoms are mirro images

Properties of optical isomers

• they have identical physical and chemical properties but with 2 exceptions:

• They rotate the plane of polarised light in opposite directions ie they are optically active

• they interact differently with other optically active molecules. They are sterioselective.

• Optical isomers interact with biological sensors in different ways

  • For example, one enantiomer of carvone smells of spearmint, while the other smells of caraway

how to determine the identity of an optical isomer of a single substance

• The rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance

  • For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance

  • Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise by a fixed number of degrees

what mechanisms do primary halogenoalkanes and tertiary halogenoalkanes use to react?

• Primary halogenoalkanes react via Sn2 mechanisms

• Tertiary halogenoalkanes react via Sn1 mechanisms

what are SN1 and SN2 mechanisms?

• SN1: Substitution, nucleophile, 1=unimolecular so 1 molecules in RDS

• SN2: substitution, nucleophilic, 2=biomolecular so 2 molecules in rds

what is a racemix micture

• A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer

  • One enantiomer rotates light clockwise, the other rotates light anticlockwise

• A racemic mixture is optically inactive as the enantiomers will cancel out each others effect

  • This means that the plane of polarised light will not change

how racemix mixtures used in drugs

• In the pharmaceutical industry, it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug

• Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures

• Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer

• This separation process is very expensive and time consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active

• For example, the pain reliever ibuprofen is sold as a racemic mixture

are SN1 mechanisms optically active?

• the reactant is optically active but product is not

• In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X^- ion

• This leaves a trigonal planar, tertiary carbocation 

• In the second step, the planar, tertiary carbocation is attacked by the nucleophile

• The OH- attacks the intermediate randomly from the right or the left. An equal number of each optical isomer is formed which results in racemix mixture

is SN2 mechanism optically active?

• The S_N2 mechanism is a one-step reaction

  • The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond

  • At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond 

  • The halogen leaves the halogenoalkane as an X^- ion

• Optically active at start to optically, but in the oposite direction

• The bromine atom of the bromoethane molecule causes steric hindrance

• This means that the hydroxide ion nucleophile can only attack from the opposite side of the C-Br bond

  • Attack from the same side as the bromine atom is sometimes called frontal attack

  • While attack from the opposite side is sometimes called backside or rear-side attack

• As the C-OH bond forms, the C-Br bond breaks causing the bromine atom to leave as a bromide ion

  • As a result of this, the molecule has undergone an inversion of configuration

  • The common comparison for this is an umbrella turning inside out in the wind

• products optically active but in the opposite directions

what are carbonyls?

• Functional group C=O

• Aldehydes and ketones and functional groups

• Aldehydes have C=O at end of a chain

• Ketones have C=O not on first/last, in middle

intermolecular forces of carbonyls

• Aldehydes and ketones have a dipole within their structure due to the electronegative oxygen of the carbonyl group. C=O bond is polar.

• This means that aldehydes and ketones have permanent dipole-dipole interactions and London forces between molecules

distinctive proprties of aldehydes and ketones

• they have distinctive smells

boiling points of aldehydes and ketones?

• Aldehydes and ketones have lower melting and boiling points than alcohols with a similar mass

• they have permanent dipole-permanent dipole interactions, rather than hydrogen bonds

• However compared to alkanes with a similar mass, their boiling points will be higher, as pd-pd forces are stronger than London forces

Solubility of carbonyls?

• Although carbonyls cannot hydrogen bond with themselves, they do have the lone pairs on their oxygen 

• This means that smaller aldehydes and ketones are able to hydrogen bond with water

  • The δ– oxygen atom from the carbonyl uses its lone pairs to form hydrogen bonds with the δ+ hydrogen from water

• As a result short chain carbonyls are soluble in water

• but longer the chain length, the less soluble it becomes

• the hydrocarbon chains form London forces that require more energy to be overcome.

• Larger aldehydes and ketones have longer hydrocarbon chains which also cannot hydrogen bond with water

  • These hydrocarbon chains can disrupt the hydrogen bonding within water but cannot form hydrogen bonds themselves

what is needed for substance to dissolve

• In order to dissolve, the strength of the potential hydrogen bonding of the carbonyl with water must be higher than the combined strength of the intermolecular forces of the carbonyl and the hydrogen bonding of water