Organic Chemistry Flashcards
What is a chiral molecule?
An asymmetrical molecule that is non-superimposable on its mirror image.
It ha a chiral carbon with 4 different groups bonded to it.
How does a racemic mixture form?
A racemic mixture is a 1:1 mixture of equal enantiomers that have no overall effect on plane-polarised light.
This forms when there is a planar centre (with carbonyl the molecule is planar around the C=O) or intermediate that can be attacked from above or below the plane.
A mixture with no overall optical activity is produced.
Define and give examples of:
a) Structural isomerism
b) Stereoisomerism
a) Structural isomerism: same molecular formula but different structural formula (positional, chain)
b) Stereoisomerism: Same structural formula but different arrangements of atoms in space (Geometric - lack of free rotation around bond, Optical - asymmetrical chiral compounds)
What is a polarimeter used for?
Measuring the extent of rotation of plane-polarised light when monochromatic light is shone through.
What is a racemic mixture?
A 50:50 mixture of equal enantiomers. There is no overall effect on plane-polarised light. Optically inactive.
What is the shape of carbonyl compounds and why?
Trigonal planar (CHO group). The C=O bond is polar so it allows nucleophiles to attack from above or below the plane, making it more susceptible to attack.
For carbonyl compounds, describe and explain:
a) the boiling points
b) the solubility
a) there are permanent dipole - permanent dipole and London forces so the boiling points are higher than alkanes but lower than alcohols.
b) they can form hydrogen bonds with water molecules (with oxygen). Smaller molecules are more soluble in water
How can you prepare aldehydes and ketones from alcohols?
Aldehydes: primary alcohol + K2Cr2O7 in H2SO4. Collect product as it distils.
Ketones: secondary alcohol + K2Cr2O7 in H2SO4. Heat under reflux.
Orange to green colour change.
In the addition reaction of carbonyl compounds to form hydroxynitriles,
a) what reagents and conditions are required and why?
b) why must the pH not be too high or too low?
c) what hazard does HCN have?
d) is the product optically active?
a) HCN with KCN: HCN is a weak acid and produces few CN- ions so KCN is added to supply sufficient CN- ions
b) (pH 8 provides CN-) Too low: few CN- ions; too high: few H+ ions
c) HCN is a toxic gas (fume cupboard)
d) optically inactive: racemic mixture; planar CHO; CN- attacks above/below
What test can be used to identify carbonyl compounds? Give the result.
a) what type of reaction is this?
b) what is the reagent acting as?
c) why is this reagent used instead of other ones?
d) state the stages involved which confirm the identity of the product.
React with Brady’s reagent (2,4-dinitrophenylhydrazine), supplied wet to reduce risk of explosion. Orange precipitate produced (insoluble)
a) Condensation (addition-elimination)
b) A nucleophile - lone pair on nitrogen
c) Brady’s reagent is less soluble so it crystallises out more easily
d) React with Brady’s reagent and filter, recrystallise, then measure melting temp and compare to data book values
What tests can be used to distinguish between aldehydes and ketones? Give the result of each test.
- Fehlings solution (Benedict’s): blue solution -> red precipitate
2Cu2+ +RCHO + OH- + H2O > Cu2O + RCOOH + 3H+ - Tollens reagent: silver mirror forms
2Ag+ + RCHO + H2O > 2Ag + RCOOH + 2H+ - Dichromate: Orange -> green colour change for aldehyde only
How is Tollens’ reagent made?
Silver nitrate is dissolved in aqueous ammonia and silver (I) oxide ppt forms.
This dissolves in more ammonia to form the diamminesilver (I) ion.
AgNO3 + NH3 > Ag2O
Ag2O + 2NH3 > [Ag(NH3)2]+
What can be used to identify methyl ketones, ethanal, and methyl alcohols?
a) give the observation made
b) give the overall equation
Iodoform reaction (Iodine and NaOH)
a) Yellow precipitate produced
b) CH3COR + 3I2 + 4NaOH > RCOONa + CHI3 + 3NaI + 3H2O
Describe and explain the physical properties of carboxylic acids:
a) Melting and boiling points
b) Solubility
c) Acidity
a) high melting and boiling points due to hydrogen bonding. Methyl and ethyl acids have higher points due to formation of dimers (2 acids joined)
b) soluble in water due to hydrogen bonding with water using OH and C=O
c) acids dissolve in water to form H+ and carboxylate ion. A delocalisation of negative charge stabilises the ion.
Give the equation for the preparation of carboxylic acids from:
a) oxidation of alcohols
b) hydrolysis of nitriles
a) ROH + 2[O] > RCOOH + H2O
Acidified K2Cr2O7 under reflux
b) RCN + 2H2O + H+ > RCOOH + NH4+
Excess HCl under reflux
