1. (2 points each) Complete the following statements or definitions. The M + 2 p
ID: 693272 • Letter: 1
Question
1. (2 points each) Complete the following statements or definitions. The M + 2 peak of an organic molecule has a relative abundance of 98% compared to the molecular ion. This compound must have the following heteroatom: A B. The best solvents for Sy2 reactions are: H S0 C. The last step of a free radical reaction is called: intahoA D Tebut ho -2 E. In the mass spectrum, the peak that corresponds to the unfragmented cation radical is molerular ion ak called F. The complex region of the intrared spectrum from 1500-400 em (right side of the spectrum) is called fingerprint region G. Infrared spectroscopy provides information about the diffevent molecules attached in an organie moleculs H Draw the carbocation (show the charge) which is the most likely to be produced by fragmentation of the original radical cation in the Mass Spectrum of the compound shown below:Explanation / Answer
A) Natural bromine, Br occurs as two isotopes having atomic masses 79 amu (Br-79) and 81 amu (Br-81). The two isotopes of Br occur in almost 50:50 ratio in nature. The MS for the given compound shows the M+2 peak having a relative abundance of 98% with respect to the molecular ion (M) peak. By convention, we shall approximate the molecular ion peak M to have a relative abundance of 100%. Since the M and the M+2 peaks show almost 1:1 relative abundance, the compound most likely contains bromine, Br.
B) SN2 reactions are favored by solvents which can stabilize the transition state electronically. Since there is a charge separation in the transition state, hence, the solvents must be polar to stabilize the transition state by dipolar interactions. The base used in a SN2 reaction is an anion; such bases can abstract protons from solvent and lose their nucelophilicity. Hence, the solvents must be devoid of protons on heteroatoms (protons on carbon atoms are not likely to be abstracted by bases). Therefore, the best solvents for SN2 reactions are polar aprotic solvents like acetone, DMSO, acetonitrile, etc.
C) The last step of a free radical reaction is called as termination. Termination is the step where all free radicals combine to form molecules. The system can contain no free radicals at the termination step and only neutral molecules are present.
D) SN1 reactions proceed via carbocation intermediates. Carbocations are electron-deficient species and are hence, stabilized by electron-donating substituents. The greater the number of electron-donating substituents, the more is the stabilization of the carbocation and hence, higher is the probability of reaction via the SN1 pathway. Consequently, a tertiary alkyl group, which has three alkyl substituents on the carbon undergoing SN1 reaction is the best alkyl group for SN1 reactions. Therefore, the answer is 3° or tertiary alkyl group.
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