Topic 17 Organic 2 Notes Flashcards
No. 1 requirement for optical isomerism?
All 4 substituents on the central carbon must be different.
What name is given to the 2 stereoisomers of a carbon with 4 different substituents?
Enantiomers.
Enantiomers
Non-superimposable mirror images of each other.
Chiral centre
The carbon with 4 different groups attached. Often labelled with *. A molecule can have more than one chiral centre.
What is special about a chiral molecule?
It can rotate the plane of polarised light.
Optically active
Term given to chiral compounds because they rotate the plane of polarised light.
Achiral compounds
Optically inactive.
Why are racemic mixtures/racemates optically inactive?
For every molecule in the mixture that rotates the plane of polarisation in one direction, there is an enantiomer that rotates the plane in the opposite direction.
Why does light emerge from a racemic mixture with its plane of polarisation unchanged?
There is an equal concentration of both enantiomers, so the rotations cancel out.
Wedge-&-dash notation
The wedge comes towards you, out of the plane. The dashed line goes away from you, into the plane. The two straight lines are next to each other.
Why is SN1 unimolecular?
It has only one species in the rate equation.
Why is SN2 bimolecular?
It has 2 species in the rate equation.
Chloride nucleophile + butanol (secondary alcohol) by SN1
Cl - can approach the cation from either side as the molecule is planar. Thus, both enantiomers form; the race mate is not optically active.
Chloride ion + butanol (secondary alcohol) by SN2
Rear-side attack, the chloride attacks the partially positive chiral carbon. Inversion occurs, and the Cl switches with the OH group. Only one enantiomer is formed, so the product is optically active.
Carbonyls
Aldehydes & ketones.
-ate
Almost always 3 or 4 oxygens.
-ide
Just the element.
Break down the name: phosphide
Reduced form of phosphorous.
Break down the name: phosphate
Oxidised form of phosphorous.
How to find the name from the formula
Use oxidation states, and work backwards from the acid.
Why do tertiary molecules react via SN1?
It forms a more stable carbocation by the positive inductive effect.
Aldehyde functional group
R-CHO
Carbonyl functional group
R-COR’
Under carefully-controlled conditions, what are primary alcohols converted into?
Aldehydes.
What are primary alcohols converted into, if there is an excess of oxidant?
Carboxylic acids.
Reduction
Add H. Add electrons. Lose O.
What are secondary alcohols converted into?
Ketones.
Why can’t tertiary alcohols be oxidised?
There is no H on the carbon to which the OH is bonded.
Oxidising agent
Acidified (H2SO4) potassium dichromate. K2Cr2O7. Cr2O7 -2/ H+.
Acidified potassium dichromate colour change
Orange to green.
Why do aldehydes have lower boiling points than alcohols?
Alcohols form H bonds, whereas aldehydes have only London dispersion forces. This means aldehydes can be distilled off as they form.
Oxidation of a primary alcohol: technique to obtain an aldehyde
Distillation
Oxidation of a primary alcohol: technique to obtain a ketone
Heating under reflux.
Alternative oxidising agent (not typically used for alcohol oxidation)
KMnO4 /H2SO4.
Potassium permanganate: colour change
Purple to pale pink.
Reducing agents
NaBH4 sodium borohydride, LiAlH4 Lithium aluminium hydride IN DRY ETHER.
What is the colour change of Fehling’s solution when warmed with an aldehyde?
Blue to red. The solution contains blue Cu (II) (aq), which is reduced to a red precipitate of Cu (I) oxide.
What happens to the aldehydes when warmed with Fehling’s solution?
It is oxidised to a. A Carboxylic acid.
Why do Tollen’s reagent and Fehling’s solution have no effect on ketones?
Ketones cannot be oxidised.
Tollen’s reagent
Alkaline ammoniacal silver nitrate solution. Silver nitrate is dissolved in aqueous ammonia to form Tollen’s reagent, incorporating [Ag(NH3)2]+ diamine silver (I) ion.
What happens to the aldehyde when warmed with Tollen’s reagent?
The aldehyde is oxidised to a carboxylic acid, and the silver (I) ions are reduced to silver metal. 2Ag+ + RCHO + H2O —> 2Ag + RCOOH + H+. A silver mirror is formed.
Reduction of aldehydes with sodium tetrahydridoborate (III) NaBH4
When dissolved in ethanol, sodium tetrahydridoborate reduces aldehydes to primary alcohols, e.g., ethanal + 2[H] — NaBH4 in ethanol—> ethanol. Occurs by Nucleophilic addition.
Reaction of ketones with NaBH4
When dissolved in ethanol, NaBH4 reduces ketones to secondary alcohols by Nucleophilic addition.
Reduction of carbonyls using lithium aluminium hydride LiAlH4 (a more powerful reducing agent)
LiAlH4 must be used dry/dissolved in ether. Aldehydes are reduced to primary alcohols. Ketones are reduced to secondary alcohols. Occurs by Nucleophilic addition, e.g., propanone + 2[H] —> propan-2-ol.
Another name for the iodoform test
Triiodomethane reaction.
Iodoform
Carbonyl + I2 in the presence of an alkali. A positive test indicates a carbonyl + a methyl group, forming an insoluble yellow precipitate and an antiseptic smell. E.g., ethanal + iodine —> triodoethanal. Triodoethanal + alkali (NaOH) —> triiodomethane.
Which compounds give a positive result for the iodoform test?
Organic compounds with a methyl group next to a carbonyl group, or secondary alcohols which will oxidise under these conditions to form a methyl group next to a carbonyl group.
Nucleophilic addition of hydrogen cyanide to aldehydes & ketones
HCN is dissolved in KCN (aq). This provides CN - ions which can attack from either side of the planar carbonyl molecule, forming a racemate. An extra carbon is added to the organic molecule, an alcohol is formed, and the CN- nucleophile regenerated.
Reaction of carbonyls with 2,4-DNP in an alcoholic solution
Forms a red/orange precipitate. Positive for both ketones and aldehydes. E.g., aldehyde + 2,4-DNP in alcoholic solution —> …-anal 2,4-dinitrophenylhydrazine by Nucleophilic addition. The product reacts with H2SO4, and a water molecule is eliminated.
What is the significance of the products of the reaction of 2,4-DNP being mostly crystalline solids?
Easy to handle. The melting points are different enough to be able to distinguish between the various carbonyl compounds. Several of the pure carbonyl compounds have very similar melting points.
Why is the carbon bonded to the oxygen in a carbonyl group susceptible to Nucleophilic attack?
The oxygen is more electronegative than the carbon meaning there is a partial positive charge on the carbon.p and a partial negative charge on the oxygen.
