Organic Chem

What is a racemic mixture?

A racemic mixture contains an equal amount of enantiomers

(enantiomers have a chiral carbon with no internal plane of symmetry. they have a pair of molecules that have non-superposable mirror images)

Explain how a racemic mixture is formed.

The carbocation intermediate has a trigonal planar shape about its a-carbon which allows the nucleophile to attack it from either side, with equal probability to form an equal amount of enantiomers

Why is the configuration of a-carbon inversed after the Sn2 reaction?

The nucleophile attacks the chiral carbon from the rear-side, causing the breaking of the C-X bond and the formation of the C-Nu bond. This results in an inverse of configuration about the a-carbon

Describe the Sn2 mechanism. Halogen: Br, Nucleophile: OH-

The :OH- nucleophile rear-sides attacks the a-carbon. A C-OH bond begins to form while the C-Br bond begins to break. This takes place via a single step via the transition state. If the halogenoalkane has a chiral a-carbon, there will be an inversion of configuration about it.

What is the rate reaction for Sn2 mechanism?

rate = k [HX][Nu]

What is the rate reaction for the Sn1 mechanism?

rate = k[HX]

Describe mechanism of Sn1. Halogen: Br, Nucleophile: OH

1. In the slow step, C-Br bond cleaving to form a carbocation intermediate. The EDG alkyl groups attached to the a-carbon help to stabalise the carbocation intermediate.

2. In the fast step, as the carbocation intermediate is trigonal planar about the a-carbon, the :OH- nucleophile can attack the a-carbon from either side with equal probability, to form equal amounts of enantiomers (if chiral carbon)

Why do halogenoalkanes undergo nucleophilic substitution?

The halogen attached to the a-carbon is electronegative, causing a partial positive charge on a-carbon. This makes the a-carbon more susceptible to nucleophilic attacks as nucleophiles are electron-rich

Why halogenoarenes cannot undergo N.S?

The p orbital of the halogen overlaps with the pi electron cloud of the benzene ring.

The lone pair of electrons from the halogen is delocalised into the benzene ring, forming a partial double bond character in the C-X bond which makes it stronger than a C-X bond in halogenoalkanes.

What is the reagents and conditions for the distinguishing test for halogen derivatives?

AgNO3 in ethanol, heat

R-Cl will form white ppt

R-Br will form cream ppt

R-I forms yellow ppt

Why does reactivity of halogens decrease down the group?

Down the group,

• the size of the halogen atoms become bigger

• this causes the C-X orbitals to become more diffuse which results in a less effective overlap

• Less energy is req to break the C-X bonds

Alcohol → Halogenoalkanes

Type of reaction, reagents, conditions

[N.S]

• PCl5, r.t (produces white fumes of HCl)

• PCl3 / SOCl2, heat

• Conc HCl, ZnCl2 catalyst, heat

Halogenoalkanes → Alcohol

Type of reaction, reagents, conditions

[N.S]

NaOH / KOH in ethanol, heat

Write out balanced equation for Idoform test. What are the reagents and conditions + observations?

R-CH(OH)CH3 + 4I2 + 6NaOH → R-COO-Na+ + CHI3 + 5 NaOH + 5NaI

I2(aq) in NaOH, heat

Brown solution decolourises and pale yellow ppt is formed

What mechanism do carbonyls undergo?

N.A

What is the rate reaction of carbonyls?

rate = k[carbonyl][:CN-]

Why must LiAlH4 be used in dry conditions and at r.t?

It reacts violently in water

No heating as it is thermally unstable

Why LiAlH4 and NaBH4 both cannot reduce C=C?

Both provide :H- nucleophiles which attack electron-deficient C atoms but C=C is electron-rich.

Why LiAlH4 is stronger RA than NaBH4?

Al is less electronegative than B and H and less electronegative atoms are more likely to give up electrons. Thus, H+ is more readily produced when LiAlH4 is used.

What is the role of HCN? (carbonyls)

Bronsted-Lowry acid (proton donor) or a Lewis Acid in Step 2 of mechanism for carbonyls

Why is cold condition needed when HCN with trace amounts of NaOH is used?

Prevent HCN poisonous gas from escaping to environment

Why do aldehydes undergo nucleophilic addition more rapidly than ketones?

Electronic factor:

Ketones have 1 more EDG alkyl group attached so the carbonyl carbon is less electron-deficient

Steric factor:

Ketones have 1 more bulky akyl group attached which hinders the approach of nucleophiles to the carbonyl carbon

Why are phenols more acidic than alcohols?

Alcohols have EDGs which intensifies the negative charge on the O atom, which destabalises the CB and decreases the acidity

In phenols, the p orbital of the O atom overlaps with the pi electron cloud of the benzene ring. Lone pair of electrons from the O atom are delocalised into the benzene ring which disperses the negative charge from the O atom and increases stability of its CB, increasing its acidity.

Reagents and conditions of reduction of nitrile

LiAlH4 in dry ether, rt

OR H2(g), Ni, rt (not heat!)