10. HALOGEN DERIVATIVES- part 06 - Mechanism of SN reaction : M

10. HALOGEN DERIVATIVES- part 06 - Mechanism of SN reaction :

Mechanism of SN reaction :

• In a nucleophilic substitution reactions of alkyl halides the halogen atom gets detached from the carbon and a new bond is formed between that electrophilic carbon and nucleophile.
• Covalently bonded halogen is converted into halide ion (X ). 
• Two electrons constituting the original covalent bond are carried away by the halogen along with it. 
• The halogen atom called ‘leaving group’.
• Leaving group is the group which leaves the carbon by taking away the bond pair of electrons. 
• The substrate undergoes two changes during a SN reaction.
• The original C-X bond undergoes heterolysisand a new bond is formed between the carbon and the nucleophile using two electrons of the nucleophile. 
• These changes may occur in one or more steps. 
• The description regarding the sequence and the way in which these two changes take place in SN reaction is called mechanism of SN reaction. 
• The mechanism is deduced from the results of study of kinetics of SN reactions. 
• Two mechanisms are observed in various SN reactions - 

1. SN¹ mechanism and 
2. SN² mechanism.

a. SN² mechanism : 

• The reaction between methyl bromide and hydroxide ion to give methanol follows asecond order kinetics.
• The rate of this reaction depends on concentration of two reacting species, namely - 

1. Methyl bromide and 
2. Hydroxide. 

• Hence it is called subtitution nucleophilic bimolecular, SN².

                                   CH3Br + OH → CH3OH + Br
                                       rate = k [CH3Br] [OH ]

• Rate of a chemical reaction is influenced by the chemical species taking part in the slowest stepof its mechanism. 
• In the above reaction only two reactants are present and both are found to influence the rate of the reaction.
  
• This means that the reaction is a single step reactionwhich can also be called the slow step. 
• This further implies that the two changes, namely, bond breaking and bond forming at the carbon take place simultaneously. 

Salient features of SN² mechanism :

1. Single step mechanism with simultaneous bond breaking and bond forming.
2. Backside attack of nucleophile : This is to avoid steric repulsion and electrostatic repulsion between the incoming nucleophile and the leaving group.
3. In the transition state (T.S.) the nucleophile and leaving groups are bonded to the carbon with partial bonds and carry partial negative charge. Thus, the total negative charge is diffused.
4. The T.S. contains pentacoordinate carbon havingthree σ (sigma) bonds in one plane making bond angles of 120⁰ with each other and two partial covalent bonds along a line perpendicular to this plane.
5. WhenSN²reaction is brought about at chiral carbon the product is found to have opposite configuration compared to that of the substrate. 
6. In other words, SN² reaction is found to proceed with inversion of configuration. This is like flipping of an umbrella. It is known as Walden inversion. 
7. The inversion in configuration is the result of backside attack of the nucleophile.

b. SN¹ Mechanism :

• The reaction between tert-butyl bromide and hydroxide ion to give tert-butyl alcohol follows a first-order kinetics.
• The rate of this reaction depends on concentration of only one species, which is the substrate molecule, tert-butyl bromide. Hence it is called substitution nucelophilic unimolecular, SN¹.
  

• Concentration of only substrate appears in the rate equation; concentration of the nucleophile does not influence the reaction rate. 
• In other words, tert-butyl bromide reacts with hydroxide by a two step mechanism.
• In the slow step C-X bond in the substrate undergoes heterolysis and in the subsequent fast step the nucleophile uses its electron pair to form a new bond with the carbon undergoing change.
  

Salient features of  SN¹mechanism :

1. Two step mechanism.
2. Heterolyis of C-X bond in the slow and reversible first step to form planar carbocation intermediate.
3. Attack of the nucleophile on the carbocation intermediate in the fast second step to form the product.
4. When  SN¹ reaction is carried out at chiral carbon in an optically active substrate, the product formed is nearly racemic.
5. This indicates that  SN¹ reaction proceeds mainly with racemization. This means both the enantiomers of product are formed in almost equal amount. 
6. Racemization in  SN¹ reaction is the result of formation of planar carbocation intermediate. 
7. Nucleophile can attack planar carbocation from either side which results in formation of both the enantiomers of the product.

Factors influencing  SN¹ and   SN² mechanism :
a. Nature of substrate : 
 SN² : 

• The T.S. of  SN² mechanism is pentacoordinateand thus crowded.
• As a result SN² mechanism is favoured in primary halides and least favoured in tertiary halides.

SN¹ :A planar carbocation intermediate is formed in SN¹ reaction.

• It has no steric crowding. 
• Bulky alkyl groups can be easily accommodated in planar carbocation See. 
• As a result SN¹ mechanism is most favoured in tertiary halides and least favoured in primary halides.
  
• Secondly the carbocation intermediate is stabilized by +I effect of alkyl substituents and also by hyperconjugation effect of alkyl substituents containing α-hydrogens. 
• As a result, SN¹ mechanism is most favoured in tertiary halides and least favoured in primary halides. 
  

• Tertiary halides undergo nucleophilic substitution by SN¹ mechanism while primary halides follow SN² mechanism. 
• Secondary halides react by either of the mechanism or by mixed mechanismdepending upon the exact conditions.

b. Nucleophilicity of the reagent :

• A nucleophile is a species that uses its electron pair to form a bond with carbon.
• Nucleophilic character of any species is expressed in its electron releasing tendency, which can be corelated to its strength as Lewis base.
•  A more powerful nucleophile attacks the substrate faster and favours SN² mechanism.
• The rate of SN¹ mechanism is independent of the nature of nucleophile. 
• Nucleophile does not react in slow step of SN¹. It waits till the carbocation intermediate is formed, and reacts fast with it.

c. Solvent polarity : 

• SN¹ mechanism proceeds via formation of carbocation intermediate.
• A good ionizing solvent, polar solvent, stabilizes the ions by solvation. 
• Solvation of carbocation is relatively poor and solvation of anion is particularly important. 
• Anions are solvated by hydrogen bonding solvents, that is, protic solvents. Thus SN¹ reaction proceeds more rapidly in polar protic solvents than in aprotic solvents.
• Polar protic solvents usually decrease the rateof SN²reaction. 
• In the rate determining step of SN² mechanism substrate as well as nucleophile is involved. 
• A polar solvent stabilizes nucleophile (one of the reactant) by solvation. Thus solvent deactivates the nucleophile by stabilizing it. 
• Hence aprotic solvents or solvents of low polarity will favour SN²mechanism.

Do you know ?

1. Negatively charged nucleophile is more powerful than its conjugate acid. For example R-O is better nucleophile than R-OH.
2. When donor atoms are from same period of periodic table, nucleophilicity decreases from left to right in a period. For example H₂O is less powerful nucleophile than NH₃.
3. When donor atoms are from same group of the periodic table, nucleophilicity increases down the group. For example, I is better nucleophile than Cl .

5 Elimination reaction :
Dehydrohalogenation

• When alkyl halide having at least one β-hydrogen is boiled with alcoholic solution of potassium hydroxide, it undergoes elimination of hydrogen atom from β-carbon and halogen atom from α - carbon resulting in the formation of an alkene. This reaction is called β-elimination (or 1,2 - elimination) reaction.
• As hydrogen and halogen is removed in this reaction it is also known as dehydrohalogenation reaction.
  
• If there are two or more non-equivalent β-hydrogen atoms in a halide, then this reaction gives a mixture of products. 
• Thus, 2-bromobutane on heating with alcoholic KOH gives mixture of but-1-ene and but-2- ene.
  
• The different products of elimination do not form in equal proportion. 
• After studying a number of elimination reactions, Russian chemist Saytzeff formulated an empirical rule given below.

1. In dehydrohalogenation reaction, the preferred product is that alkene which has greater number of alkyl groups attached to doubly bonded carbon atoms.
2. Therefore, in the above reaction but-2-ene is the preferred product, and is formed as the major product. 
3. It turned out that more highly substituted alkenes are also more stable alkenes.Hence Saytzeff elimination is preferred formation of more highly stabilized alkene during an elimination reaction. 
4. The stability order of alkyl substituted alkenes is

                  R2C = CR2 > R2C = CHR > R2C = CH2,

                       RCH = CHR > RCH = CH2

Remember...

• Carbon bearing halogen is commonly called α-carbon (alpha carbon) and any carbon attached to α-carbon is β-carbon (beta carbon). 
• Hydrogens attached to β-carbon are β-hydrogens.

Do you know ?

Elimination versus substitution:

• Alkyl halides undergo sunstitution as well as elimination reaction. 
• Both reactions are brought about by basic reagent, hence there is always a competition between these two reactions. 
• The reaction which actually predominates depends upon following factors.

a. Nature of alkyl halides : 

• Tertiary alkyl halides prefer to undergo elimination reaction where as primary alkyl halides  prefer to undergo substitution reaction.

b. Strength and size of nucleophile :

• Bulkier electron rich species prefers to act as base by abstracting proton, thus favours elimination. 
• Substitution is favoured in the case of comparatively weaker bases, which prefer to act as nucleophile

c. Reaction conditions : 

• Less polar solvent, high temperature fovours elimination where as low tempertaure, polar solvent favours substitution reaction.