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

• 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 otheroptically active molecules. They are sterioselective.

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

are SN1 mechanisms optically active?

• the reactant is optically active but product is not

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

is SN2 mechanism optically active?

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

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