Sn1 reaction

Topics: Nucleophilic substitution, Hydrochloric acid, SN1 reaction Pages: 5 (620 words) Published: October 29, 2014
Reminder: These notes are meant to supplement, not replace the laboratory manual.

SN1 Reaction Notes
Background and Application
Substitution Nucleophilic First Order (SN1) reactions are one of the most common type of organic reactions. SN1 reactions can be used to make a wide variety of new compounds. In this experiment, t-amyl alcohol will be converted by a SN1 mechanism to 2-chloro-2-methylbutane.

Safety Precautions
Concentrated Hydrochloric Acid is 12M. It will cause visible destruction of tissue upon contact. Wear eye goggles at all times during this experiment. Keep HCl away from skin, clothes, eyes. Neutralize any spilled HCl with sodium bicarbonate (baking soda) and wipe up immediately.

Gasses will be released when sodium bicarbonate is added to your separatory funnel. Do not stopper funnel after the addition of sodium bicarbonate solution. Vent separatory funnel frequently while shaking. Do not vent sep funnel toward another person or yourself. Liquids may spray out while venting. Chlorinated hydrocarbons tend to be toxic materials. The target organs are the liver and kidneys. Be careful not to breathe product or get on skin.

1. Some terminology related to SN1 Reactions:
Synthesis: A reaction which results in the formation of a new product. Mechanism: A step by step description of bond breaking and bond breaking as one compound is transformed into another compound.

Leaving Group: This is the part of a molecule which may break away, taking the electrons from the bond with it. The more stable the leaving group as an independent assemblage, the more readily it will depart the molecule. Carbocation: A carbon with only 6 shared valence shell electrons. A full positive charge resides on the carbon. Carbocations are electrophiles.

Nucleophile: An atom or assemblage of atoms which will donate electrons to form a new covalent bond. Strong nucleophiles are strong Lewis bases.
Electrophile: An atom which will accept electrons and form a new covalent bond. Electrophiles have either a formal positive charge or a partial positive charge. They have a low electron density and hence will accept electrons from a nucleophile.

SN1 mechanism: A two step Nucleophilic substitution mechanism which is rate dependent only on the starting alkyl concentration. K=k[R-LG]. The first step, the leaving group leaves forming a carbocation. This is the slow rate determining step (RDS). In the second step, a nucleophile forms a new covalent bond with the electrophile.

2. This is a synthetic experiment. Today each of you will make a new compound by reacting two starting materials together. 2-methyl-2-butanol (t-amyl alcohol or t-pentyl

alcohol) is converted to 2-chloro-2-methyl butane (t-amyl chloride or t-pentyl chloride). The overall reaction is as follows.

3. This reaction is carried out in the presence of excess concentrated hydrochloric acid. HCl is highly corrosive. Goggles and gloves should be worn at all times. Care should be taken to not get acid on skin. Hydrochloric acid is a strong acid (pKa = - 3.9) which means the acid completely disassociates.

4. Most chlorinated organic materials have increased toxicity to humans. The target organs are usually the liver and kidney. Minimize direct contact with these materials. 5. SN1 reactions are nucleophilic substitution reaction with first order kinetics in regard to the organic substrate. This is a three-step mechanism. First the acid protonates the alcohol to form a good leaving group; then water (the leaving group) departs, forming a carbocation. This is the rate determining step (RDS). The last step is a Lewis acid-base reaction in which the nucleophile combines with the carbocation to form the product. The entire mechanism is depicted below.

6. 2-methyl-2-butanol is soluble in water. 2-chloro-2-methylbutane is insoluble in water (
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