e-Assignment #2

Free Radical Chlorination of 1-chlorobutane

Free radical halogenation was explored in Chem 501 in terms of its mechanism, enthalpy, and reactivity. In terms of the mechanism, I learned that the free radical halogenation involves three steps: Initiation, Propagation, and Termination.

Step	Description
Initiation	Heat, light or an initiatior induce homolytic cleavage of the halogen molecule to produce to radicals.
Propagation	Consists of two steps: the radical is consumed in the first and produced in the second making it a catalyst; involves the production of the desired product(s).
Termination	Stops propagation by removing the radical necessary for it to continue; cut down on the efficiency of the overall reaction.

Once the mechanism was agreed upon, enthalpy became the focus of class discussions. Using bond dissociation energies and Hess's Law, the enthalpy, delta sign

H, for each individual propagation step was calculated as well as an overall enthalpy change. Understanding how enthalpy is calculated and the process for doing so, enabled me to understand more thoroughly why some reactions are considered to be favorable and others are not. More specifically, a favorable(spontaneous) reaction depends on the relationship between enthalpy and free energy. Typically, the free energy change, delta sign

G, of a reaction is calculated according to the following mathematical expression: delta sign

G =

H - T

S. However, there is not a noticable change in entropy, delta sign

S, during a free radical chlorination. This simplifies the expression for free energy to the following: delta sign

G =

H. Overall, a negative value for delta sign

H will produce a negative delta sign

G, which indicates a favorable reaction in terms of thermodynamics.
    At this point, I could predict whether or not a free radical halogenation would occur, but I could not explain which hydrogen on the starting material would be abstracted and replaced by the halogen radical. To remedy this problem, primary hydrogens were compared to secondary hydrogens in terms of their relative reactivity and chemical kinetics was discussed. By the end of the POGIL, I learned that secondary hydrogens are typically more reactive than primary hydrogens and the propagation step with the highest transistion energy state constitutes the rate-determining step.

    The Chem 502 prelab discussion and the experiment itself reiterated and expanded on the concepts covered in Chem501. In the lab, I was able to apply my understanding of the relative reactivity of primary and secondary hydrogens and analyze how the trend can be altered by the presence of a halogen on the starting material. Below is an excerpt from my final lab report detailing the conclusions I gathered from the experiment.

        "The chlorination of 1-chlorobutane produced four different dichlorinated products. They were 1,1-dichlorobutane, 1,2-dichlorobutane, 1,3-dichlorobutane and 1,4-dichlorobutane. These products were analyzed by gas chromatography in order to determine a trend among the relative reactivity of hydrogen atoms on the 1-chlorobutane. It was apparent from the experiment’s results that 1,3-dichlorobutane was more readily produced from the starting material, which indicates more reactive hydrogen atoms on the third carbon in 1-chlorobutane. In particular, the 1,3-dichlorobutane was 8 times more likely to be produced then the 1,1-dichlorobutane.

Typically, primary hydrogen atoms are not as reactive as secondary hydrogen atoms. However, this trend is not followed explicitly due to the chlorine substituent on the starting material. The presence of the chlorine substituent should increase the strength of the C-H bonds close to it, which would lower the hydrogen atom’s relative reactivity.¹ Therefore, the 1,1-dichlorobutane product would be expected to be produced the least, and the results confirm the theory.

Additionally, steric hindrance plays a part in determining the percent composition of each product. The chlorine substituent already present in the starting material will make it much more difficult for another chlorine atom to bond to the same carbon. It will also interfere with formation of a C-Cl bond on the second carbon in the chain. It will be less likely to affect the formation of the C-Cl bond on the third and fourth carbons. This reasoning would indicate a higher percentage of the 1,4-dichlorobutane product than the 1,2-dichlorobutane; however, this is not evident in the gas chromatogram. Therefore, there must be another factor in determining the relative reactivity of the hydrogen atoms.

The last factor is most likely the stability of the carbon radical that forms during the free-radical process. A radical will be more stable when there are more R groups attached to the carbon it is on. The 1,4 product requires are radical to be present on the last carbon atom in the chain, which will be less stable than a radical present on the second carbon in the chain. This is why the 1,2-dichlorobutane is more readily produced when compared to the 1,4-dichlorobutane.

In conclusion, when attempting to determine the relative reactivity of hydrogen atoms present on monochlorinated starting materials, several factors must be considered. These factors include the location of the chlorine substituent, steric hindrance, and radical stability."

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