Question

Like Markovnikov, Saytzeff states his rule in terms, not of product stability, but of numbers of hydrogens on carbon atoms, (a) Suggest a wording for this original Saytzeff rule, (b) Predict the major product of dehydrohalogenation of 2-bromo-1-phenylbutane on the basis of the original rule, (c) On the basis of the modern rule.

Short answer

(a) The original Saytzeff rule states: in an elimination reaction, the major product is the one formed by the removal of hydrogen from the β-carbon having the fewest hydrogens. (b) For 2-bromo-1-phenylbutane, the major product using the original Saytzeff rule is 1-phenyl-1-butene, formed by dehydrohalogenation at the carbon next to the phenyl group. (c) Using the modern Saytzeff rule, which focuses on product stability, the major product is also 1-phenyl-1-butene, as it has more alkyl substituents around the double bond compared to 1-phenyl-2-butene.

Step-by-step solution

(a) Suggest a wording for the original Saytzeff rule)

The original Saytzeff rule, also known as the "Zaitsev's rule", can be stated as follows: In an elimination reaction, the major product is the one formed by the removal of hydrogen from the β-carbon having the fewest hydrogens.

(b) Predict the major product of dehydrohalogenation of 2-bromo-1-phenylbutane using the original Saytzeff rule)

To predict the product using the original Saytzeff rule, we first need to draw the structure of 2-bromo-1-phenylbutane, and identify the β-carbons for the elimination reaction. The structure of 2-bromo-1-phenylbutane is: \( \mathrm{CH_2PhCH(Br)CH_2CH_3} \). There are two β-carbon atoms available for the elimination reaction: 1. The carbon next to the phenyl group, which has 1 hydrogen. 2. The carbon at the end of the chain, which has 3 hydrogens. Now, using the original Saytzeff rule, we choose the β-carbon with the fewest hydrogens. In this case, it is the carbon next to the phenyl group. After the dehydrohalogenation reaction, the product formed will be an alkene with a double bond between the carbon next to the phenyl group and the carbon with the bromine group. The major product in this case is 1-phenyl-1-butene.

(c) Predict the major product of dehydrohalogenation of 2-bromo-1-phenylbutane using the modern Saytzeff rule)

The modern Saytzeff rule focuses on the stability of the products formed after the elimination reaction. In this case, the major product is the one that results in the most stable alkene. The two possible β-carbon atoms for the elimination reaction are still the same as mentioned above. The two potential products are: 1. 1-phenyl-1-butene (from the first β-carbon) 2. 1-phenyl-2-butene (from the second β-carbon) Now, in order to determine the stability of these products, we need to consider the number of alkyl substituents around the double bond. Alkenes with more alkyl substituents are more stable. 1-phenyl-1-butene has 2 alkyl substituents around the double bond: a phenyl group and a methyl group. 1-phenyl-2-butene has only 1 alkyl substituent around the double bond: a phenyl group. Based on this, 1-phenyl-1-butene is more stable than 1-phenyl-2-butene, and therefore, the major product according to the modern Saytzeff rule is 1-phenyl-1-butene, which is the same as the result obtained using the original Saytzeff rule.

Key concepts

Dehydrohalogenation

Dehydrohalogenation is a chemical reaction that involves the elimination of a hydrogen halide (HX, where X is a halogen) from an organic compound. The reaction is a type of elimination reaction, which inherently means that two atoms or groups are removed from a molecule, resulting in the formation of a double bond. In organic chemistry, this reaction is particularly important as a method to generate alkenes from alkyl halides.

The general reaction mechanism for dehydrohalogenation includes the abstraction of a proton (H⁺) by a base from a carbon atom adjacent (beta position) to the carbon bearing the halogen. This leads to the formation of a double bond between the two carbons as the halide ion leaves, producing an alkene and a halogen acid as products.

An example of this process is when 2-bromo-1-phenylbutane undergoes dehydrohalogenation to form 1-phenyl-1-butene. A strong base such as potassium hydroxide (KOH) would typically be used to abstract the hydrogen and a solvent like ethanol facilitates the reaction.

Elimination Reaction

An elimination reaction, in the context of organic chemistry, is a type of reaction where two substituents are removed from a molecule in such a way that a pi bond is formed. This transformation generally occurs between two consecutive atoms, leading to the formation of multiple bonds such as double or triple bonds.

There are two common types of elimination reactions: E1 and E2. E1 stands for a unimolecular elimination and involves a two-step mechanism where the halide leaves first to form a carbocation intermediate. E2, or bimolecular elimination, is a one-step process where the base abstracts the proton and the leaving group departs simultaneously, leading to more concerted and often favored outcomes.

It is essential to note that the presence of a good leaving group and a suitable base are critical to driving the elimination reaction forward. In dehydrohalogenation, the elimination reaction is a type of E2 where we see a hydrogen halide being removed to form a new alkene.

Alkene Stability

Alkene stability is a significant concept when predicting the outcome of elimination reactions such as dehydrohalogenation. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, and their stability is influenced by the substitution pattern of the double bond.

The more substituted an alkene is, the more stable it typically becomes. This is due to the hyperconjugation effect where the sigma bonding electrons in the carbon-hydrogen bonds of neighboring carbon atoms can delocalize into the adjacent pi bond system, effectively increasing the stability of the alkene.

For instance, tetrasubstituted alkenes are generally more stable than trisubstituted, which are more stable than disubstituted, and so on. In the context of the Saytzeff rule, this principle leads to the prediction that the most substituted (and thus most stable) alkene product will be favored in an elimination reaction.

1-Phenyl-1-butene

1-Phenyl-1-butene is an organic compound and a specific example of an alkene that can be synthesized through dehydrohalogenation. The compound consists of a phenyl group attached to the first carbon of the butene chain, which implies the presence of a double bond between the first and second carbon atoms in the butyl moiety.

Reflecting on alkene stability, 1-phenyl-1-butene benefits from the stability offered by the phenyl group which acts as a powerful electron-releasing group through resonance stabilization. Additionally, in the example of the dehydrohalogenation of 2-bromo-1-phenylbutane, the Saytzeff rule prioritizes the formation of 1-phenyl-1-butene due to its more stabilized nature, with the phenyl and methyl substituents providing stability to the resultant double bond.

Thus, 1-phenyl-1-butene is not only a product of dehydrohalogenation but also an exemplification of the Saytzeff rule's preference for the formation of more substituted, and thereby more stable, alkenes.