2- Bromopropane protons

Figure 13.15 The 1H NMR spectrum of 2-bromopropane. The -CH3 proton signal at 1.71 <5 is split into a doublet, and the -CHBr- proton signal at 4.28 8 is split into a septet. Note that the distance between peaks—the coupling constant—is the same in both multiplets. Note also that the outer two peaks of the septet are so small as to be nearly lost.

Mechanism. The double bond ti electrons attack the electrophile. Protonation of the double bond yields a secondary carbocation inter mediate. The bromine nucleophile attacks the carbocation to form 2-bromopropane. 

The H NMR spectrum of 2-bromopropane. The -CHj proton signai at 1.71 is split into a doublet, and the -CHBr- proton signal at 4.28 S is split into a septet. 

An alkyl halide such as 2-bromopropane has two j8-carbons from which a proton can be removed in an E2 reaction. Because the two j8-carbons are identical, the proton can be removed with equal ease from either one. The product of this elimination reaction is propene. 

Protons in the same environment are called chemically equivalent protons. For example, 1-bromopropane has three different sets of chemically equivalent protons. The toee methyl protons are chemically equivalent because of rotation about the C—C bond. The two methylene protons on the middle carbon are chemically equivalent, and the two methylene protons on the carbon bonded to the bromine atom make up the third set of chemically equivalent protons. 

Each set of chemically equivalent protons in a compound gives rise to a signal in the NMR spectrum of that compound. (Sometimes the signals are not sufficiently separated and overlap each other. When this happens, one sees fewer signals than anticipated.) Because 1-bromopropane has three sets of chemically equivalent protons, it has three signals in its NMR spectrum. 

Bromopropane has two sets of chemically equivalent protons and, therefore, it has two signals in its NMR spectrum. The six methyl protons in 2-bromo-propane are equivalent, so they give rise to only one signal. Ethyl methyl ether has three sets of chemically equivalent protons the methyl protons on the carbon adjacent to the oxygen, the methylene protons on the carbon adjacent to the oxygen. 

Ethoxide and malonate anions have virtually identical proton basicity (9), but significantly different hardness. As a consequence, ethoxide promotes elimination of 2-bromopropane, whereas malonate effects substitution. 

Entry 5 is an example of a stereospecific anti elimination reaction of an alkyl halide in which the transition state requires the proton being removed and the bromide lost to be in an anti orientation with respect to each other. The dias-tereomeric threo- and eryfhro-l,2-diphenyl-l-bromopropanes undergo base-catalyzed jS-elimination to produce stereoisomeric products. Entry 6 is an example of pyrolytic elimination requiring a syn orientation of the proton being removed and the amine oxide nitrogen atom. This elimination reaction is a concerted one in which the proton is abstracted by the oxygen of the amine oxide group. 

The polyethers containing three, five and seven methylene units were analyzed by 200 MHz spectroscopy. The last two polymers present bromo-alkane chain ends only. A typical NMR spectrum is shown in Figure 2 together with its proton assignments. The polyethers of 1,3-dibromopropane and HMS present both bromopropane and allyl ether chain ends. A typical spectrum is given in Figure 3. The ratio between allyl ether and bromopropane chain ends depends on the reaction conditions used. If a small... 

The technically most important electrophilic addition reactions proceed at alkenes and alkynes. The reactive rr-electrons of these compounds are attacked by electrophiles, resulting in the formation of a positively charged reaction intermediate. Stabilization of this positive charge plays a very important role in the regioselectivity of electrophilic addition reactions. This is demonstrated in Scheme 2.2.11 for the addition of HBr to propene, a reaction that produces almost uniquely the product 2-bromopropane and almost no 1-bromopropane as a consequence of the inductive stabilization of the secondary propyl cation compared to the cation with the charge at Cl. This selctivity, where the proton becomes attached to the carbon with fewer alkyl substituents, is known as Markovnikov s rule. 

Here are proton NMR data for 1-bromopropane Hai triplet (2H) 3.32ppm Hbi multiplet (2H) l.Slppm Ho triplet (3H) 0.93ppm. (Relative integrations shown in parentheses.)... 

Upon very close inspection of the proton NMR spectrum of 1-bromopropane, you would find that peak cluster b has at least six peaks. Is this consistent with your answer in part d) ... 

---