organic mechanisms

ORGANIC MECHANISMS : 

ORGANIC MECHANISMS Curly arrows etc.

ALKANES – Homolytic free radical substitution. : 

ALKANES – Homolytic free radical substitution. Initiation Cl Cl UV in sunlight provides energy for the Cl – Cl bond to split homolytically. The result is two chlorine free radicals (atoms with unpaired electrons)

Propogation 1 : 

Propogation 1 A chlorine free radical collides with a methane molecule. A second homolytic fission results in a new molecule [HCl] and a new free radical Cl H H H H C

Propogation 2 : 

Propogation 2 A methyl free radical collides with a chlorine molecule A third homolytic fission results in the formation of chloromethane and regenerates a chlorine free radical Propogation steps 1 and 2 can continue alternating in a chain reaction Cl Cl

Termination : 

Termination Two free radicals collide and form a molecule. This ends the chain reaction Cl

ALKENES – Heterolytic electrophilic addition. : 

ALKENES – Heterolytic electrophilic addition. Ethene bromination Bromine molecule approaches the double bond and is polarised by the high electron density pi orbitals Pi electrons form a concentrated and exposed region of negative charge d+ d-

STAGE 1; formation of a carbonium ion intermediate. : 

STAGE 1; formation of a carbonium ion intermediate. The electron pair from the pi bond move to join the d+ bromine atom onto one of the double bond carbons This leaves the other carbon atom electron deficient so it becomes a positive carbonium ion (carbocation) The electron pair from the bromine molecule sigma bond both move to the d- bromine atom to make a free bromide ion (Br -) C C H H H H Br Br d+ d-

STAGE 2; the carbonium intermediate and the bromide ion combine : 

STAGE 2; the carbonium intermediate and the bromide ion combine This stage happens quickly so the intermediate is short lived. C H H C H H Br Br

Here is another animation of the same mechanism : 

Here is another animation of the same mechanism

HALOGENOALKANES. Nucleophilic substitution; SN1/SN2 : 

HALOGENOALKANES. Nucleophilic substitution; SN1/SN2 SN1 mechanism. Unimolecular. Tertiary halogenoalkanes. The polar C – Hal bond breaks with the electron pair moving to the Halogen atom. The carbonium ion which is left is the intermediate. C CH3 CH3 H3C Cl d- d+ INTERMEDIATE The intermediate is quickly attacked by the nucleophile OH- The product is a tertiary alcohol OH

HALOGENOALKANES. Nucleophilic substitution; SN1/SN2 : 

HALOGENOALKANES. Nucleophilic substitution; SN1/SN2 SN2 mechanism. Bimolecular. Primary halogenoalkanes. The OH- nucleophile approaches the d+ carbon from the opposite side to the d- halogen C Cl CH3 H H d+ d- OH An electron pair forms a new bond between OH- and d+ carbon, at the same time the C – Cl bond lengthens and weakens A negative carbanion is formed but it is a TRANSITION STATE, not an INTERMEDIATE as it is merely the highest energy point in a transition and has no independent life of its own

HALOGENOALKANES. Nucleophilic substitution; SN1/SN2 : 

HALOGENOALKANES. Nucleophilic substitution; SN1/SN2 SN2 mechanism. Bimolecular. Primary halogenoalkanes. Second phase The HO – C bond continues to shorten and strengthen as the Cl leaves as a chloride ion, taking with it the negative charge brought by hydroxide. C Cl CH3 H H OH There is INVERSION of the molecule. This is a ONE STEP process involving collision between TWO molecules, [while SN1 is a TWO STEP mechanism initiated by break up of ONE molecule]

Here is another animation of the SN2 mechanism : 

Here is another animation of the SN2 mechanism

CARBONYL COMPOUNDS; Nucleophilic addition : 

CARBONYL COMPOUNDS; Nucleophilic addition The carbonyl group is polar Cδ+ is attacked by a nucleophile such as CN- A nucleophile such as CN- will attack here.

The nucleophilic addition mechanism : 

CN The nucleophilic addition mechanism C O R1 R2 The nucleophile attacks the δ+ carbon atom. A bond is formed and the electron pair in the π bond between O and C move onto the oxygen atom. A negative charged intermediate is formed which rapidly reacts with H+ ions to make a hydroxynitrile δ+ δ- H