Reac 714 Studying Sn1 and Sn2 Reactions: Nucleophilic Substitution at Saturated Carbon
REAC 714 Studying SN1 and SN2 Reactions: Nucleophilic Substitution at Saturated Carbon
Date of Experiment: February 6, 2008
Objective:
The objective of this laboratory experiment is to study both SN1 and SN2 reactions. The first part of the lab focuses on synthesizing 1-bromobutane from 1-butanol by using an SN2 mechanism. The obtained product will then be analyzed using infrared spectroscopy and refractive index. The second part of the lab concentrates on how different factors influence the rate of SN1 reactions. The factors that will be examined are the leaving group, Br versus Cl-; the structure of the alkyl group, 3◦ versus 2◦; and the polarity of the solvent, 40 percent 2-propanol versus 60 percent 2-propanol.
Discussion/Theory/Reaction Mechanisms
SN2 Reaction Mechanism:
This reaction takes place by an SN2 mechanism. The reaction must be carried out in a very acidic solution because the OH group of the alcohol is not a weak base, and therefore, not a good leaving group. By conducting the reaction in a very acidic environment, the alcohol group is converted into water, which is a weak base, and thus a good leaving group. Due to lack of steric interference, the primary carbon atom is now open to back-side attack. The bromide ion, which acts as the nucleophile attacks from the back-side and bonds to the carbon atom. As a result, 1-bromobutane is formed and water is given off as the leaving group.
SN1 Reaction Mechanism:
The reactions that take place in the second part of the experiment all take place by way of an SN1 reaction. All of the reactions proceed with the same mechanism. SN1 reactions occur under solvolysis conditions, where the solvent serves as the nucleophile.
The slow, rate-determining step occurs as the bromine leaves the 2-bromo-2-methylpropane, forming a 3◦ carbocation. The water, which acts as the nucleophile and the solvent, attaches to the carbocation at the central carbon. Another water molecule then removes a
Date of Experiment: February 6, 2008
Objective:
The objective of this laboratory experiment is to study both SN1 and SN2 reactions. The first part of the lab focuses on synthesizing 1-bromobutane from 1-butanol by using an SN2 mechanism. The obtained product will then be analyzed using infrared spectroscopy and refractive index. The second part of the lab concentrates on how different factors influence the rate of SN1 reactions. The factors that will be examined are the leaving group, Br versus Cl-; the structure of the alkyl group, 3◦ versus 2◦; and the polarity of the solvent, 40 percent 2-propanol versus 60 percent 2-propanol.
Discussion/Theory/Reaction Mechanisms
SN2 Reaction Mechanism:
This reaction takes place by an SN2 mechanism. The reaction must be carried out in a very acidic solution because the OH group of the alcohol is not a weak base, and therefore, not a good leaving group. By conducting the reaction in a very acidic environment, the alcohol group is converted into water, which is a weak base, and thus a good leaving group. Due to lack of steric interference, the primary carbon atom is now open to back-side attack. The bromide ion, which acts as the nucleophile attacks from the back-side and bonds to the carbon atom. As a result, 1-bromobutane is formed and water is given off as the leaving group.
SN1 Reaction Mechanism:
The reactions that take place in the second part of the experiment all take place by way of an SN1 reaction. All of the reactions proceed with the same mechanism. SN1 reactions occur under solvolysis conditions, where the solvent serves as the nucleophile.
The slow, rate-determining step occurs as the bromine leaves the 2-bromo-2-methylpropane, forming a 3◦ carbocation. The water, which acts as the nucleophile and the solvent, attaches to the carbocation at the central carbon. Another water molecule then removes a
References: Jerry Manion. Studying SN1 and SN2 Reactions: Nucleophilic Substitution at Saturated Carbon, Chemical Education Resources, 1998, H.A. Neidig pg 1-16. http://orgchem.colorado.edu/hndbksupport/irtutor/tutorial.html Dr. Shaffer Sabrina Howard