Passage
Two key ingredients found in many soaps are PEG 150 distearate and ethylhexyl glycerin. Both can be synthesized from glyceryl tristearate (Figure 1) .
Figure 1 Formation of PEG 150 distearate and ethylhexyl glycerin from glyceryl tristearateThe first step in this process requires the saponification of glyceryl tristearate. The resulting stearate molecules can then react with PEG 150 in an acid-catalyzed Fischer esterification to form PEG 150 distearate. The glycerol produced during saponification is used as a nucleophile to form ethylhexyl glycerin through a base-catalyzed S_N2 reaction with an ethylhexyl halide.Studies have shown that hand washing with soap can substantially reduce the number of germs present on hands and that washing hands before preparing food is particularly effective in reducing illness. Researchers examined the effect of washing for 20 seconds with different amounts of soap containing PEG 150 distearate and ethylhexyl glycerin and found an inverse correlation between the volume of soap used and the amount of bacteria recovered from the hands. The data are shown in Figure 2.
Figure 2 Bacteria recovered from hands after washing with varying amounts of soapThe researchers then investigated the effect of hand washing with different kinds of soap on the transfer of bacteria from hands to objects. Volunteers washed with a fixed volume of soap for 30 seconds, then handled sterilized plastic spheres for 30 seconds. Bacteria were then recovered from the spheres. The results are shown in Table 1.Table 1 Bacteria Recovered from Objects
Adapted from Fuls JL, Rodgers ND, Fischler GE, et al. Alternative hand contamination technique to compare the activities of antimicrobial and nonantimicrobial soaps under different test conditions. Appl Environ Microbiol. 2008.
-Why does the bond between glycerol and the ethylhexyl group tend to form at glycerol's C1 hydroxyl group instead of the C2 hydroxyl group?

Question

Passage
Two key ingredients found in many soaps are PEG 150 distearate and ethylhexyl glycerin. Both can be synthesized from glyceryl tristearate (Figure 1) .

Figure 1 Formation of PEG 150 distearate and ethylhexyl glycerin from glyceryl tristearateThe first step in this process requires the saponification of glyceryl tristearate. The resulting stearate molecules can then react with PEG 150 in an acid-catalyzed Fischer esterification to form PEG 150 distearate. The glycerol produced during saponification is used as a nucleophile to form ethylhexyl glycerin through a base-catalyzed S_N2 reaction with an ethylhexyl halide.Studies have shown that hand washing with soap can substantially reduce the number of germs present on hands and that washing hands before preparing food is particularly effective in reducing illness. Researchers examined the effect of washing for 20 seconds with different amounts of soap containing PEG 150 distearate and ethylhexyl glycerin and found an inverse correlation between the volume of soap used and the amount of bacteria recovered from the hands. The data are shown in Figure 2.

Figure 2 Bacteria recovered from hands after washing with varying amounts of soapThe researchers then investigated the effect of hand washing with different kinds of soap on the transfer of bacteria from hands to objects. Volunteers washed with a fixed volume of soap for 30 seconds, then handled sterilized plastic spheres for 30 seconds. Bacteria were then recovered from the spheres. The results are shown in Table 1.Table 1 Bacteria Recovered from Objects

Adapted from Fuls JL, Rodgers ND, Fischler GE, et al. Alternative hand contamination technique to compare the activities of antimicrobial and nonantimicrobial soaps under different test conditions. Appl Environ Microbiol. 2008.
-Why does the bond between glycerol and the ethylhexyl group tend to form at glycerol's C1 hydroxyl group instead of the C2 hydroxyl group?

Quizplus · Accepted Answer

11ecba2d_1266_6e0e_a3bf_fba12834fe0b_MD0008_00 Steric hindrance can prevent reactions that would otherwise occur by physically blocking the access of one atom to another. Alcohols may be attached to primary, secondary, or tertiary carbon atoms, which are bonded to one, two, or three other carbons, respectively. These alcohols are referred to as primary, secondary, and tertiary alcohols. A tertiary alcohol is more sterically hindered than a secondary alcohol because it has more bonds to large atoms that can get in the way of interactions with other molecules. A secondary alcohol is more hindered than a primary alcohol for the same reason. The alcohol on carbon 2 of glycerol is a secondary alcohol whereas carbon 1 is a primary alcohol. The added steric hindrance at carbon 2 prevents S_N2 reactions from happening as readily as they do at carbon 1. Therefore, the C1 hydroxyl group reacts with an ethylhexyl halide more easily than the C2 hydroxyl group does. (Choice A) Secondary alcohols are more likely than primary alcohols to undergo S_N1 reactions. However, the reaction of interest is an S_N2 reaction, for which primary alcohols are more efficient. (Choice C) Primary alcohols are more acidic than secondary alcohols. This attribute contributes to their enhanced ability to react because deprotonation by a base makes the alcohol a better nucleophile. (Choice D) The alcohol is acting as a nucleophile, so enhanced electrophilicity would make it a worse reactant. Educational objective: Sterically hindered chemical groups are physically inhibited from interacting with other molecules. For this reason, they are less capable of acting as nucleophiles in S_N2 reactions. Tertiary alcohols are more sterically hindered than secondary alcohols, which are more sterically hindered than primary alcohols.

Passage Two Key Ingredients Found in Many Soaps Are PEG 150