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Chapter 12: Problem 65

Answer true or false. (a) Phenols and alcohols have in common the presence of an \(-\) OH group. (b) Phenols are weak acids and react with strong bases to give water-soluble salts. (c) The \(\mathrm{p} K_{\mathrm{a}}\) of phenol is smaller than that of acetic acid.

Short Answer

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(a) True, (b) True, (c) False.

Step by step solution

01

Analyzing Statement (a)

Phenols and alcohols are both organic compounds that contain a hydroxyl group (-OH). However, in phenols, the -OH group is directly attached to an aromatic benzene ring, while in alcohols, it is attached to a saturated carbon atom in an alkyl group. Despite this difference, both contain the -OH group, making the statement true.
02

Evaluating Statement (b)

Phenols are known to be weak acids due to the acidic hydrogen on the hydroxyl group. When phenols react with strong bases like sodium hydroxide (NaOH), they form water-soluble phenoxide salts. This is a characteristic property of phenols, thus making the statement true.
03

Comparing pKa Values for Statement (c)

The strength of an acid is often discussed in terms of its pKa value, where a lower pKa indicates a stronger acid. The typical pKa of phenol is around 10, whereas acetic acid has a pKa around 4.75. This means acetic acid is the stronger acid compared to phenol, and therefore, the pKa of phenol is larger than that of acetic acid. This makes the statement false.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Phenols
Phenols are a type of organic compound characterized by the presence of a hydroxyl group (-OH) attached directly to an aromatic hydrocarbon group, specifically a benzene ring. This structural difference imparts unique properties to phenols, distinguishing them from other compounds that also contain hydroxyl groups. Phenols exhibit characteristic behaviors that both students and chemists find noteworthy.
  • Acidity: One of the defining features of phenols is their ability to act as weak acids. The acidic nature is due to the donation of the hydrogen ion from the hydroxyl group. This property is enhanced by the stability of the phenoxide ion formed after the loss of a proton. The aromatic ring allows resonance stabilization in the phenoxide ion, making the ion more stable compared to other hydroxyl group-containing species.
  • Reactivity with Bases: Phenols can react with strong bases like sodium hydroxide (NaOH) to form phenoxide salts, which are water-soluble. This chemical reaction highlights another of phenol鈥檚 important traits, as it contrasts with alcohols, which generally do not display similar acidic behavior in aqueous solutions.
Alcohols
Alcohols are organic compounds in which a hydroxyl group (-OH) is attached to a saturated carbon atom, typically within an alkyl group. This structural configuration leads to alcohols displaying properties distinct from phenols despite their shared functional group.
  • General Structure: In alcohols, the hydroxyl group binds to a carbon atom not involved in an aromatic system, resulting in different chemical properties compared to phenols. This configuration tends to make alcohols less acidic than phenols.
  • Physical Properties: Alcohols often act as versatile solvents due to their ability to engage in hydrogen bonding, which can influence their solubility and boiling points. The hydrogen bonding capability results in higher boiling points for alcohols compared to hydrocarbons of similar molecular weight.
  • Chemical Reactions: Alcohols participate in a variety of chemical reactions such as dehydration, esterification, and oxidation. These reactions showcase their versatility and usefulness in chemical synthesis and industry.
Hydroxyl Group
The hydroxyl group is a simple, but highly significant functional group in organic chemistry identified as -OH. Its presence in various organic molecules dictates their chemical and physical behavior, making it a key concept worth understanding.
  • Role in Acidity: The hydroxyl group contains an oxygen atom, which is more electronegative than hydrogen. This makes it capable of forming hydrogen bonds and in specific conditions, can donate a hydrogen ion, conferring acidic properties to a molecule. This role is particularly noteworthy in phenols where the -OH contributes to the acidity.
  • Solubility: The ability to form hydrogen bonds influences the solubility of compounds containing the hydroxyl group. Molecules with more hydroxyl groups tend to be more soluble in water due to increased hydrogen bonding with water molecules.
  • Implications in Different Classes of Compounds: The presence of hydroxyl groups in both phenols and alcohols demonstrates its widespread impact across various classes of organic compounds. In each, the -OH group impacts molecular behavior significantly, from acidity to solubility.

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Most popular questions from this chapter

Suppose you have unlabeled bottles of benzene and cyclohexene. What chemical reaction could you use to tell which bottle contains which chemical? Explain what you would do, what you would expect to see, and how you would explain your observations. Write an equation for a positive test.

cis-3-Hexene and trans-3-hexene are different compounds and have different physical and chemical properties. Yet, when treated with \(\mathrm{H}_{2} \mathrm{O} / \mathrm{H}_{2} \mathrm{SO}_{4},\) each gives the same alcohol. What is this alcohol, and how do you account for the fact that each alkene gives the same one?

Answer true or false. (a) Benzene does not undergo the addition reactions that are characteristic of alkenes. (b) A defining feature of aromatic compounds is that they are highly unsaturated but do not undergo characteristic alkene addition reactions. (c) Nitration of benzene adds a \(-\mathrm{NO}_{2}\) group to one of the carbons of the aromatic ring. (d) Halogenation of an alkene is an addition reaction; halogenation of an arene is a substitution reaction.

In omega-3 fatty acids, the last carbon of the last double bond of the hydrocarbon chain is three carbons from the methyl terminal end of the chain. The last carbon of the chain is called the omega carbon, hence, the designation omega-3. Eicosapentaenoic acid is a common omega-3 fatty acid found in cold water fatty fish and health food supplements. \\[ \mathrm{CH}_{3}\left(\mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}\right)_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H} \\] Eicosapentaenoic acid a \(\mathrm{C}_{20}\) polyunsaturated fatty acid (a) How many cis-trans isomers are possible for this fatty acid? (b) Draw a line-angle formula for eicosapentaenoic acid, showing the cis configuration of all carboncarbon double bonds in the hydrocarbon chain.

Three products with the molecular formula \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{BrCl}\) form when bromobenzene is treated with chlorine, \(\mathrm{Cl}_{2}\) in the presence of \(\mathrm{FeCl}_{3}\) as a catalyst. Name and draw a structural formula for each product.
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