Question

Treating 4-penten- 1 -ol with bromine in water forms a cyclic bromoether. Account for the formation of this product rather than a bromohydrin as was formed in Problem \(6.33 .\)

Short answer

Short Answer: Treating 4-penten-1-ol with bromine in water results in the formation of a cyclic bromoether instead of a bromohydrin because the reaction mechanism favors the formation of more stable, less strained cyclic intermediates. The nucleophilic attack of water on the more substituted carbon of the bromonium ion leads to a six-membered ring, which is more stable than the five-membered ring that would result if water attacked the less substituted carbon.

Step-by-step solution

Understanding the reactant structure

Firstly, let's analyze the structure of the reactant 4-penten-1-ol. It is an alcohol (pentanol) with a double bond (4-pentene) between the 4th and 5th carbon. The structure is as follows: \[CH_2=CHCH_2CH_2CH_2OH\]

Initial reaction with bromine

When 4-penten-1-ol reacts with bromine, the double bond quickly reacts with the bromine molecule, forming a bromonium ion. This intermediate has a positive charge on the bromine atom, with the two carbon atoms bonded to it sharing this positive charge: \[CH_2Br - CHBr - CH_2CH_2CH_2OH\]

Nucleophilic attack by water

In the presence of water, a nucleophilic attack takes place. Water acts as a nucleophile and attacks the more substituted carbon (carbon 4) of the bromonium ion. This results in the formation of a cyclic bromoether: \[CH_2Br - CH - CH_2CH_2OCH_2CH_2Br\]

Explanation for cyclic bromoether formation

In this reaction, the nucleophilic attack by water on the more substituted carbon leads to a six-membered ring intermediate. The formation of six-membered ring intermediates is favored due to lower ring strain. If the water attacked the less substituted carbon, it would result in a five-membered ring intermediate, which would be less stable. This drives the reaction towards the formation of the cyclic bromoether rather than a bromohydrin. In conclusion, treating 4-penten-1-ol with bromine in water results in the formation of a cyclic bromoether due to the reaction mechanism, which favors the formation of more stable, less strained cyclic intermediates.

Key concepts

Alcohol Reactivity

Exploring the reactivity of alcohols, such as 4-penten-1-ol, reveals some intriguing characteristics. Alcohols consist of an -OH group attached to a carbon chain and display a range of reactivity based on the structure around the hydroxyl (-OH) group. With 4-penten-1-ol, the hydroxyl group is located at the end of the chain, making it a primary alcohol.

Primary alcohols, like our example, are generally less reactive than secondary and tertiary alcohols because of the lower carbon substitution around the hydroxyl group. Despite this, primary alcohols can still engage in chemical reactions, particularly those influenced by other functional groups in the molecule. In this case, the double bond close to the alcohol functional group enhances its reactivity. This double bond is poised to participate in reactions, such as the addition of a bromine molecule, leading to the formation of various intermediates and, subsequently, different products depending on the reaction conditions.

Bromonium Ion Intermediate

The chemistry behind the formation of a bromonium ion intermediate is central to understanding how cyclic bromoethers are made. When the double bond within 4-penten-1-ol interacts with bromine, the electrons in the double bond are used to form a bond with the bromine atom. This interaction gives rise to a bromonium ion intermediate, a three-membered ring where the positive charge is distributed over the two carbon atoms and the bromine.

The bromonium ion is highly reactive due to its positive charge and strained ring structure, making it an ideal target for nucleophiles, such as water in this case. Its formation is a key step in the reaction, setting the stage for the nucleophilic attack that leads to the production of cyclic structures, as seen in the creation of bromoethers from alcohols with double bonds.

Nucleophilic Attack

Nucleophilic attack is a cornerstone of organic chemistry, an event where a molecule with a pair of electrons (a nucleophile) seeks out an electron-poor center (an electrophile) to form a bond. In the treatment of 4-penten-1-ol with bromine and water, water acts as the nucleophile, targeting the electrophilic carbon atoms within the bromonium ion intermediate.

This nucleophilic attack is not arbitrary; it follows Markovnikov's rule where the nucleophile favors the more substituted carbon. Why? Because it's more stable due to the added alkyl groups that release electrons, making those carbons more electron-rich and a better target for the electron-poor nucleophile. Thus, in our specific exercise, the nucleophilic water molecule attacks the more substituted carbon, resulting in a six-membered cyclic bromoether which is favored due to the lower ring strain and higher stability compared to five-membered rings.