Organic 1 Lecture 9 PDF

Substitution and Elimination Reactions of Alkyl Halides  Reactions type of Alkyl Halides (R-X) Alkyl Halides (R-X) have two types of reactions: 1 2  1-Nucleophilic Substitution Reactions (SNR) Substrate Factor affecting: 1- Nature of substrate...

Substitution and Elimination Reactions of Alkyl Halides  Reactions type of Alkyl Halides (R-X) Alkyl Halides (R-X) have two types of reactions: 1 2  1-Nucleophilic Substitution Reactions (SNR) Substrate Factor affecting: 1- Nature of substrate 2- Strength of Nu Type of Substrate  The Nucleophile (Nu) A nucleophile is a reagent consisting of an atom, ion, or molecule that donates electron pair to form a bond. It is an electron-rich species that can be negatively charged or neutral with available electrons. A nucleophile reacts with an electrophile by donating electrons to form the bond Examples of Nucleophilic Substitution Mechanism of Nucleophilic Substitution Reaction  The SN2 Mechanism Mechanism H H.. H.......... :O H + C Br : H..O C Br : H C + : Br :.. H......O.. H H HH H transition state  Key Characteristics of the SN2 Mechanism 1. A one step (2o order reaction) 2. Nu attacks from the opposite side of the LG 3. Reactant undergoes inversion of configuration 4.Mechanism affected by steric hindrance (i.e. bulky or large R groups) Larger R groups will decrease the rate constant of SN2 rxns Stereochemistry in the SN2 Mechanism  The SN1 Mechanism A nucleophilic substitution reaction taking place in two steps is an example of a unimolecular nucleophilic substitution (SN1) mechanism. Step 1 Step 2  Key Characteristics of the SN1 Mechanism 1. A two step (1o order reaction) 2. Nu attacks from the top and bottom sides of the C+ intermediate 3. Reactant undergoes racemization 4. Mechanism favored by stable carbocation Example:  Reactivity of substrate in SN1 Rxns Note: Methyl and primary RX don’t undergo SN1 reactions. Stereochemistry in the SN1 Mechanism Examples of racemization in SN1  2. Elimination Reactions When a 2° or 3° alkyl halide is treated with a strong base such as NaOH, dehydrohalogenation occurs producing an alkene – via elimination reaction. 1 2 21  Saytzeff’s Rule If more than one elimination product is possible, the most- substituted alkene is the major product (most stable). H Br CH3 CH3 - H H CH3 OH H C C C CH3 C C C CH3 + H C C C H H H H H H H H CH3 minor major