to carbon #3. If we had started numbering at the other end, the bromine would be in position #4; hence the name, 3-bromohexane In the second example, there are two potential five-carbon chains; in this case the chain with the most substituents is selected as parent, (a pentane Attached to the pentane at carbon #2 is a bromo group and at carbon #3, an ethyl group; hence the name 2-bromo-3-ethy pentane In the third example, the iodine is attached to a cyclohexane ring and wil therefore be named as a halocycloalkane; the name is iodocyclohexane (no number required) In the last example, the chlorine and two methyl groups are attached to a cyclopentane ring. The numbering must begin at the carbon bearing the methy l groups since the sequence [1, 1, 2] is lower at the first point of difference than the sequence [1, 2, 2], which you would have if the chlorine was attached to carbon #l; the name is therefore 2-chloro-1, 1-dimethy l cyclopentane Nucleophilic Substitution Reactions As described previously, the carbon-halogen bond in alkyl halides is polarized placing a partial positive charge on the carbon, and a partial negative charge on the halogen. The partially positive carbon is therefore electrophilic and will be susceptible to attack by nucleophiles. When a suitable nucleophile attacks an alkyl halide, it can displace the halogen in a substitution reaction to release the halide anion and form a new bond to the carbon as shown below Leaving gro CH3 Nucleophile With simple primary alky l halides reacting with simple nucleophiles, the rate at which this substitution reaction proceeds is proportional to both the concentration of the nucleophile and the concentration of the reac tant alkyl halide, making the reaction second order. This type of second-order nucleophilic displacement reaction is therefore termed an S,2 reaction (substitution, nucleophilic, bimolecular). The mechanism for this reaction is best described as concerted with the reaction coordinate passing through a single energy maximum with no distinct intermediate. The transition state forto carbon #3. If we had started numbering at the other end, the bromine would be in position #4; hence the name, 3-bromohexane. In the second example, there are two potential five-carbon chains; in this case, the chain with the most substituents is selected as parent, (a pentane). Attached to the pentane at carbon #2 is a bromo group and at carbon #3, an ethyl group; hence the name 2-bromo-3-ethylpentane. In the third example, the iodine is attached to a cyclohexane ring and will therefore be named as a halocycloalkane; the name is iodocyclohexane (no number required). In the last example, the chlorine and two methyl groups are attached to a cyclopentane ring. The numbering must begin at the carbon bearing the methyl groups since the sequence [1,1,2] is lower at the first point of difference than the sequence [1,2,2], which you would have if the chlorine was attached to carbon #1; the name is therefore 2-chloro-1,1-dimethylcyclopentane. Nucleophilic Substitution Reactions As described previously, the carbon-halogen bond in alkyl halides is polarized, placing a partial positive charge on the carbon, and a partial negative charge on the halogen. The partially positive carbon is therefore electrophilic and will be susceptible to attack by nucleophiles. When a suitable nucleophile attacks an alkyl halide, it can displace the halogen in a substitution reaction to release the halide anion and form a new bond to the carbon, as shown below. With simple primary alkyl halides reacting with simple nucleophiles, the rate at which this substitution reaction proceeds is proportional to both the concentration of the nucleophile and the concentration of the reactant alkyl halide, making the reaction second order. This type of second-order, nucleophilic displacement reaction is therefore termed an "SN2" reaction (substitution, nucleophilic, bimolecular). The mechanism for this reaction is best described as concerted with the reaction coordinate passing through a single energy maximum with no distinct intermediate. The transition state for