Alcohols

Question 1

What suffix will a hydroxyl (-OH) group give to the name of a molecule?

Assume no higher-priority functional groups are present.

Answer 1

“-ol”

For example, ethanol is an ethane chain with a single hydroxyl group replacing one hydrogen. “Ethane-ol” gives us its name, ethanol.

Question 2

What suffix will two hydroxyl (-OH) groups give to the name of a molecule?

Assume no higher-priority functional groups are present.

Answer 2

“-diol”

For example, methanediol is a methane molecule with two hydroxyl groups replacing two hydrogens. “Methane-diol” gives us its name, methanediol.

Question 3

How is an alcohol designated in the nomenclature of a molecule that also contains higher-priority functional groups?

Answer 3

An alcohol is designated by the prefix “hydroxy”.

For example, 1-hydroxypropanone is a propane chain with a carbonyl group on its central carbon (a ketone) and a hydroxyl group on its first carbon. “1-hydroxyl-propane-ketone” gives us its name, 1-hydroxypropanone.

Question 4

What general term describes an alcohol located on the end of a carbon chain?

Answer 4

A terminal hydroxyl group is called a primary alcohol.

Primary alcohols are bound to primary carbons, which are carbons that are attached to only one other carbon atom.

For example, 1-propanol consists of a propane chain with one hydroxyl group bound to its first carbon.

Question 5

What general term describes an alcohol bound to a carbon in the middle of a non-branching chain?

Answer 5

A non-terminal hydroxyl group in the middle of a straight carbon chain must be bound to a secondary carbon. For this reason, it is called a secondary alcohol.

For example, 2-propanol consists of a propane chain with one hydroxyl group bound to its second carbon.

Question 6

What general term describes an alcohol bound to a carbon at the intersection of three carbon chains?

Answer 6

A hydroxyl group at the intersection of three carbon chains must be bound to a tertiary carbon. For this reason, it is called a tertiary alcohol.

For example, 2-methylbutanol consists of a butane chain with one hydroxyl group bound to its second carbon, which is also attached to a methyl group.

Question 7

What is the IUPAC name of the following molecule, and what type of alcohol is it?

Answer 7

This molecule is 1-propanol, a primary alcohol.

Question 8

What is the IUPAC name of the following molecule, and what type of alcohol is it?

Answer 8

This molecule is 2-butanol, a secondary alcohol.

Question 9

What is the IUPAC name of the following molecule, and what type of alcohol is it?

Answer 9

This molecule is 1,3-butanediol. As a diol, it contains two hydroxyl groups, one of which is primary and one of which is secondary.

Question 10

What is the IUPAC name of the following molecule, and what type of alcohol is it?

Answer 10

This molecule is 2-methyl-2-butanol, a tertiary alcohol.

Question 11

Which type of intermolecular attractions are alcohols, but not alkanes, able to exert?

Answer 11

Alcohols are able to undergo hydrogen bonding.

Remember that any molecule that contains a hydrogen attached to F, N, or O is able to hydrogen bond. While hydrogen bonding is stronger than dipole-dipole or London dispersion forces, it is much weaker than any intramolecular bond.

Question 12

How does the boiling point of an alcohol compare to that of an alkane of similar size?

Answer 12

The alcohol has a higher boiling point.

Alcohols undergo hydrogen bonding, while alkanes can only exert London dispersion forces. When molecular size is similar, intermolecular attractive forces determine the relative boiling point difference between two molecules.

Question 13

How does the boiling point of an alcohol compare to that of an aldehyde of similar size?

Answer 13

The alcohol will have a higher boiling point.

Alcohols undergo hydrogen bonding, while aldehydes can only exert dipole-dipole interactions. When molecular size is similar, intermolecular attractive forces determine the relative boiling point difference between two molecules.

Question 14

How does the boiling point of methanol compare to that of water?

Answer 14

Methanol has a lower boiling point than water.

Both water and methanol can undergo hydrogen bonding. However, each water molecule has two hydrogens bound to its oxygen atom, allowing it to possess twice as many hydrogen bonds than methanol per molecule. More hydrogen bonding always increases the intermolecular attractive forces, which causes the boiling point to rise.

Question 15

How does the branching of its carbon chain affect the boiling point of an alcohol?

Assume all other factors remain constant.

Answer 15

Increased branching decreases the boiling point of an alcohol.

Branching decreases boiling point in general, not just for alcohols. The added branches reduce the surface area available for attractive intermolecular forces.

Question 16

Rank the following molecules in order from lowest to highest boiling point.

• n-butanol
• isobutanol
• tert-butyl alcohol

Answer 16

1. tert-butyl alcohol (83°C)
2. isobutanol (108ºC)
3. n-butanol (118ºC).

Increased branching decreases the boiling point of any carbon-based organic compound. The highly branched tert-butyl alcohol, then, must have a significantly lower boiling point than the unbranched n-butanol.

Question 17

Rank the following molecules in order from lowest to highest boiling point.

• 2,3-dimethyl-2-butanol
• 1-hexanol
• 3-methyl-2-pentanol

Answer 17

1. 2,3-dimethyl-2-butanol (119ºC)
2. 3-methyl-2-pentanol (134ºC)
3. 1-hexanol (158ºC).

Increased branching decreases the boiling point of any carbon-based organic compound. The highly branched 2,3-dimethyl-2-butanol, then, must have a significantly lower boiling point than the unbranched 1-hexanol.

While you should be able to draw the structures of molecules like this given the names, those structures are included below for reference.

Question 18

How does the branching of its carbon chain affect the melting point of an alcohol?

Assume all other factors remain constant.

Answer 18

Between already-branched alcohols, increased branching tends to increase the melting point, as long as the branching creates a more compact structure.

Branching increases melting point in general, not just for alcohols. The added branches makes the molecule’s shape more compact and easier to stack into a solid.

Question 19

Which of these two molecules has a higher melting point?

Answer 19

Tert-buyl alcohol

Branching tends to increase the melting point, since the added branches make the shape more compact and easier to fit into a solid structure. Straight-chain (unbranched) molecules are not included in this general rule.

Question 20

When an alcohol is introduced into an acidic environment, what will result?

Answer 20

The alcohol group will be protonated.

If an environment is sufficiently acidic, it will add an additional proton to the alcohol’s oxygen atom, turning the -OH group into an -OH₂⁺ group. As a water molecule, this protonated alcohol is an effective leaving group. For this reason, protonation is the first step in many reactions that involve alcohols.

Question 21

What product will form when a tertiary alcohol reacts with a nucleophile in an acidic environment?

Answer 21

tertiary substituted product

For example, if the nucleophile was Br^-, the product will be a tertiary alkyl bromide.

The acidic environment protonates the -OH group, turning it into -OH₂⁺, which is an effective leaving group. Since the leaving group is bound to a tertiary carbon, it must dissociate spontaneously via an S_N1 reaction. This generates a carbocation, which is later attacked by the nucleophile, forming a tertiary substituted product.

Question 22

What product will form when tert-butanol reacts with HBr in an aqueous environment?

Answer 22

2-bromo-tert-butane

Since the leaving group is bound to a tertiary carbon, this reaction must occur via an S_N1 mechanism; S_N2 reactions do not occur with tertiary reactants. Bromine, the nucleophile, will simply substitute in for the -OH group.

Question 23

What is the rate law for the reaction between tert-butanol and HBr in an aqueous environment?

Answer 23

Rate = k_obs [C₄H₉OH]

Since the leaving group is bound to a tertiary carbon, this must be an S_N1 reaction. S_N1 reactions are unimolecular, meaning that they depend only on the original substrate (tert-butanol), not the nucleophile (Br^-).

Question 24

What product will form when a primary alcohol reacts with a nucleophile in an acidic environment?

Answer 24

primary substituted product

For example, if the nucleophile was Br^-, the product will be a primary alkyl bromide.

The acidic environment protonates the -OH group, turning it into -OH₂⁺, which is an effective leaving group. The electrophilic carbon bound to this group is attacked by the nucleophile, and the -OH₂⁺ is displaced via an S_N2 reaction mechanism. Note that primary substrates cannot undergo S_N1 reactions.

Question 25

What product will form when 1-propanol reacts with HBr in an aqueous environment?

Answer 25

1-bromopropane

Since the leaving group is bound to a primary carbon, this reaction must occur via an S_N2 mechanism; S_N1 reactions do not occur with primary reactants. Bromine, the nucleophile, will simply substitute in for the -OH group.

Question 26

What is the rate law for the reaction of 1-propanol with HBr in an aqueous environment?

Answer 26

Rate = k_obs [C₃H₇OH] [HBr]

Since the leaving group is bound to a primary carbon, this must be an S_N2 reaction. S_N2 reactions are bimolecular, meaning that they depend on both the original substrate (1-propanol) and the nucleophile (the Br^- in HBr).

Question 27

Name at least three compounds that are used as oxidizing agents in organic reactions.

Answer 27

• the chromium reagents (CrO₃, CrO₄^2-, Cr₂O₇^2-)
• permanganate (MnO₄^-) and manganate (MnO₄^2-)
• nitric acid (HNO₃)
• pyridinium chlorochromate (PCC)

For the MCAT, remember that oxidizing agents generally contain many oxygen atoms. Reducing agents usually contain many hydrogen atoms.

Question 28

What product(s) can form from the oxidation of a primary alcohol?

Answer 28

• aldehyde
• carboxylic acid

Broadly, oxidation involves the gain of bonds to oxygen or the loss of bonds to hydrogen. Weak oxidizing agents like PCC will only convert a primary alcohol to an aldehyde, while strong oxidizing agents like dichromate will oxidize one step further to a carboxylic acid.

Question 29

What product(s) can form from the oxidation of a secondary alcohol?

Answer 29

ketone

Broadly, oxidation involves the gain of bonds to oxygen or the loss of bonds to hydrogen. Here, two bonds on the central carbon are already used to connect to other carbons; even a very strong oxidizing agent will not be able to break these. A carboxylic acid group cannot be produced from this starting material.

Question 30

What product(s) can be formed from the oxidation of a tertiary alcohol?

Answer 30

None. The initial tertiary alcohol will remain unreacted.

Broadly, oxidation involves the gain of bonds to oxygen or the loss of bonds to hydrogen. Here, three bonds on the central carbon are already used to connect to other carbons; even a very strong oxidizing agent will not be able to break these.

Question 31

What product will form when 1-pentanol is reacted with PCC?

Answer 31

Pentanal, an aldehyde.

As a comparatively weak oxidizing agent, PCC only converts primary alcohols to aldehydes, not to carboxylic acids.

Question 32

What product will form when 1-pentanol is reacted with chromic acid (H₂CrO₄)?

Answer 32

Pentanoic acid, a carboxylic acid.

Like most of the chromium reagents (but unlike PCC), CrO₄^2- is a strong oxidizing agent. It will convert primary alcohols all the way to carboxylic acids, which are as oxidized as possible.

Question 33

What name is given to the rearrangement that can occur with a 1,2-diol in acidic conditions?

Answer 33

Pinacol rearrangement

In a Pinacol rearrangement, whichever -OH group that will yield a more stable carbocation (tertiary or secondary) is protonated and leaves. One of the alkyl chains then rearranges to take its place. This allows the remaining -OH group to form a double bond to carbon (creating a carbonyl) and lose its proton to solution.

Question 34

What final product will 2,3-methyl-butan-2,3-diol yield when placed in an acidic environment?

Answer 34

3,3-methyl-butan-2-one

Since the original molecule is a diol with hydroxyl groups on adjacent carbons, this reaction proceeds via a Pinacol rearrangement. The final product results from one of the methyl groups shifting to replace the protonated hydroxyl group that left. The remaining hydroxyl has been converted to a ketone.

Question 35

What steps must be taken to allow the final product of a multi-step reaction to retain a highly reactive group?

Answer 35

A protecting group must be added to modify the reactive functional group. This group will then be removed in a final work-up step.

For example, the reducing agent LAH converts carbonyls into alcohols. Imagine a case where an ester must be reduced without removing its carbonyl group. If the carbonyl is first converted into an acetal (a protecting group), then it will not react with LAH. The acetal can later be removed by reacting it with aqueous acid.

Question 36

An alcohol group is converted to an acetyl group, preventing it from reacting during later steps of the procedure. The alcohol group can later be recovered intact. What name is given to the initial step that occurred?

Answer 36

protection step

Protection is the conversion of a highly reactive substituent into a more stable and less reactive group, allowing the original substituent to be recovered later.

Question 37

An alcohol group is converted to an acetyl group, preventing it from reacting during later steps of the procedure. At the end of the process, the alcohol group is recovered through a reaction with a strong base. What name is given to the final step that occurred?

Answer 37

deprotection step

Deprotection is the final work-up in which the protected group is converted back into the original substituent.

Question 38

What type of product will form when thionyl chloride (SOCl₂) is reacted with an alcohol?

Answer 38

Alkyl halide (specifically, an alkyl chloride). In such a molecule, the -OH group is replaced with -Cl.

HCl and SO₂ are side products in this reaction.

Question 39

What final product(s) will be formed in the reaction below?

Answer 39

• The main product is ethyl chloride.
• The side products include HCl and SO₂.

When an alcohol is reacted with thionyl chloride, the -OH group will be replaced with -Cl, creating hydrochloric acid and sulfur dioxide as side products.

Question 40

What type of product will form when phosporus tribromide (PBr₃) is reacted with an alcohol?

Answer 40

Alkyl halide (specifically, an alkyl bromide). In such a molecule, the -OH group is replaced with -Br.

HBr and HP(O)(OH)₂ are side products in this reaction. Note that three alcohol molecules are necessary to fully react with one PBr₃ molecule.

Question 41

What final product(s) will be formed in the reaction below, assuming an excess of alcohol?

Answer 41

• The main product is ethyl bromide
• The side product is HP(O)(OH)₂.

When an alcohol is reacted with phosphorus tribromide, the -OH group on three alcohol molecules will be replaced with -Br, creating HP(O)(OH)₂ as a side product.

Question 42

What is the purpose of reacting an alcohol with thionyl chloride (SOCl₂) or phosphorus tribromide (PBr₃)?

Answer 42

This reaction results in an alkyl halide, which is very reactive. For this reason, it can be a useful initial step to perform instead of directly converting an alcohol to a different group.

Alkyl halides are especially reactive because Cl^-, Br^-, and I^- are excellent leaving groups. Note that F^-, due to its small size, is a much poorer leaving group than the other halogens.

Question 43

What type of product will form when methanesulfonyl chloride (CH₃SClO₂) is reacted with an alcohol in basic solution?

Answer 43

A mesylate ester will form. In this molecule, the -H of the -OH group is replaced with -CH₃SO₂.

Side products include the Cl^- ion and the conjugate acid of the base used.

Question 44

What final product(s) will be formed in the reaction below?

Assume the reaction takes place in the presence of a base, such as triethylamine.

Answer 44

• The main product is ethyl mesylate.
• The side products include Cl^- and the conjugate acid of the base used.

When an alcohol is reacted with methanesulfonate, a mesylate ester is formed. In such a molecule, the -OH group is replaced with -CH₃SO₂.

Question 45

What type of product will form when toluenesulfonyl chloride (CH₃C₆H₄SClO₂) is reacted with an alcohol?

Answer 45

A tosylate ester will form. In this molecule, the -OH group is replaced with -CH₃C₆H₄SO₂.

HCl will also form as a side product.

Question 46

What final product(s) will be formed in the reaction below?

Assume the reaction takes place in a base, such as triethylamine.

Answer 46

• The main product is ethyl tosylate.
• The side product is also formed as HCl.

When an alcohol is reacted with toluenesulfonyl chloride, a tosylate ester is formed. In such a molecule, the alcohol -OH group is replaced with -CH₃C₆H₄SO₂.

Question 47

What is the purpose of reacting an alcohol with toluenesulfonyl chloride (CH₃C₆H₄SO₂Cl) or methanesulfonyl chloride (CH₃ClO₂S)?

Answer 47

These reactions result in either a tosylate or a mesylate, respectively. Both compounds are very reactive, so this can be a useful initial step to perform instead of directly converting an alcohol to a different group.

These products are especially reactive because the large, stable structures of mesylates and tosylates make them excellent leaving groups.

Question 48

What reactants and products are involved in a Fischer esterification?

Answer 48

The reactants are an alcohol and a carboxylic acid in the presence of a dehydrating agent. The products are an ester and water.

This reaction can be favored by using the alcohol in excess, by using a strong dehydrating agent (such as sulfuric acid), or by physically removing water from the solution.

Question 49

What final product(s) will be formed in the reaction below?

Assume this reaction takes place in the presence of sulfuric acid.

Answer 49

• The major product is ethyl ethanoate.
• Water is also formed as a side product.

When an alcohol is reacted with a carboxylic acid and a dehydrating agent, an ester is formed. The alcohol -OH group attacks the carbonyl carbon; the proton from the alcohol is then transfered to the hydroxyl group on the carboxylic acid, which leaves as water.

Question 50

Other than carboxylic acids, which types of molecule can be used to synthesize esters?

Answer 50

Esters can be formed from acyl chlorides or acid anhydrides. These reactions occur as follows:

• An alcohol is reacted with an acyl chloride, ideally in anhydrous conditions. This produces an ester and HCl.
• An alcohol is reacted with an acid anhydride, ideally in anhydrous conditions. This produces an ester and a carboxylic acid.

Anhydrous conditions are preferred because both acyl chlorides and acid anhydrides also react with water.

Question 51

What final product(s) will be formed in the reaction below?

Assume this reaction takes place in an anhydrous environment.

Answer 51

• The main product is propyl ethanoate.
• HCl is also formed as a side product.

Remember that an alcohol reacted with an acyl chloride in an anhydrous environment will produce an ester and HCl.

Question 52

What final product(s) will be formed in the reaction below?

Assume this reaction takes place in an anhydrous environment.

Answer 52

• The main product is propyl propanoate.
• Propanoic acid is also formed as a side product.

Remember that an alcohol reacted with an acid anhydride in an anhydrous environment will produce an ester and a carboxylic acid.

Question 53

What characteristic distinguishes an inorganic ester from an organic ester?

Answer 53

• Inorganic esters form from inorganic acids.
• Organic esters form from organic acids.

In general, esters form from the condensation of an acid and an alcohol.

Specifically, an inorganic ester is an oxo compound in which the oxygen is double-bound to a heteroatom such as phosphorus, sulfur, or nitrogen.

Question 54

What type of molecule is this?

Answer 54

It is an inorganic ester, or more specifically, a phosphoric acid ester.

“Ester” is a general term for the condensation product of an acid and an alcohol. An inorganic ester simply has its “carbonyl” oxygen double-bonded to a heteroatom such as phosphorus, sulfur, or nitrogen.

Question 55

What final product(s) will be formed in the reaction below? Assume that phenol is present in excess.

Answer 55

• The main product is triphenyl phosphate ester.
• Water is also formed as a side product.

“Ester” is a general term for the condensation product of an acid and an alcohol. The -OH groups on phosphoric acid are protonated by the attacking alcohol and leave as water. This process repeats for each of the three -OH groups, until three phenyl groups have been added to form this inorganic ester.

Question 56

How does the acidity of an alcohol compare to that of other oxygen-containing compounds (such as ketones, esters, and carboxylic acids)?

Answer 56

Alcohols are more acidic than esters, aldehydes, and ketones, but less acidic than carboxylic acids.

This general trend assumes that other factors, like size and inductive effect, are similar.

Note that the most acidic hydrogen atom on esters, aldehydes, and ketones is the alpha proton.

Question 57

Rank compounds A, B, and C (below) in order from least to most acidic.

Answer 57

From least to most acidic: C, B, A.

In general, and assuming similar size/structure/induction, esters are the least acidic, followed by aldehydes/ketones and alcohols. Carboxylic acids are the most acidic of the compounds shown.