1o, 2o, and 3o Alcohols
Alcohols are classified according to the type of carbon to which the –OH group is attached. Since C atoms form four bonds, the C atom bearing the –OH group can be attached to a further 1, 2, or 3 alkyl groups, the resulting alcohols classified as primary, secondary, and tertiary alcohols (1o, 2o, and 3o).
Polyalcohols
Alcohols that contain more than one hydroxyl group are called polyalcohols; the suffixes –diol and –triol are added to the entire alkane name to indicate two and three –OH groups, respectively. The abundance of –OH groups makes it a good participant in hydrogen bonding with water, a property that makes it a valuable ingredient in skin moisturizers, hand lotions, and lipsticks, and in foods such as chocolates.
Cyclic and Aromatic Alcohols
Cyclic compounds that have –OH groups attached are called cyclic alcohols; many large molecules are known by their common names that often end in –ol. Aromatic compounds can have –OH groups attached, forming the aromatic alcohols.
Ether: An organic compound with two alkyl groups attached to an oxygen atom. The two alkyl groups may be identical (R – O – R’) or different (R’ – O – R”).
Functional group: R – O – R’
Preparation:
condensation reaction, eliminating H20; dehydration
R – OH + R’ –OH → R – O – R’ + H –OH
Properties of Ethers
Ethers are good solvents for organic reactions because they mix readily with both polar and non-polar substances. Their C – O bonds make them more polar than hydro-carbons and thus ethers are more miscible with polar substances than are hydrocarbons. Meanwhile, their alkyl groups allow them to mix readily with non-polar substances. In addition, the single covalent C – O bonds in ethers are difficult to break, making ethers quite unreactive, another property of a good solvent.
Condensation Reactions of Alcohols to Ethers
Ethers are formed when two alcohol molecules react. A molecule of water is eliminated and the remaining portions of the two alcohol molecules combine to form ether. This type of reaction, in which two molecules interact to form a larger molecule with a loss of a small molecule such as water, is called a condensation reaction.
Section 1.6 ALDEHYDES and KETONES
Aldehyde: An organic compound characterized by a terminal carbonyl functional group; that is, a carbonyl group bonded to at least one H atom.
Ketones: An organic compound characterized by the presence of a carbonyl group bonded to two carbon atoms.
Functional group: >C = O, carbonyl group
Preparation:
-
primary alcohols → aldehydes
controlled oxidation reactions
-
secondary alcohols → ketones
controlled oxidation reactions
Pathways to other groups:
-
aldehydes → primary alcohols
addition reaction with hydrogen: hydrogenation
-
ketones → secondary alcohols
addition reaction with hydrogen: hydrogenation
Properties of Aldehydes and Ketones
Aldehydes and ketones have lower boiling points than analogous alcohols, and are less soluble in water than alcohols; this is to be expected as they do not contain –OH groups and so do not participate in hydrogen bonding. However, the carbonyl group is a strongly polar group due to the four shared electrons in the double C=O bond. Thus, aldehydes and ketones are more soluble in water than are hydrocarbons. The ability of these compounds to mix with both polar and non-polar substances makes them good solvents.
Section 1.7 CARBOXYLIC ACIDS AND ESTERS
Carboxylic acid: One of a family of organic compounds that is characterized by the presence of a carboxyl group; – COOH
Ester: An organic compound characterized by the presence of a carbonyl group bonded to an oxygen atom.
Functional groups:
- carboxylic acid: – COOH carboxyl group
- ester: – COOR alkylated carboxyl group
Preparation:
-
alcohol + (O) → aldehyde + (O) → carboxylic acid
oxidation reaction; add (O)
-
carboxylic acid + alcohol → ester + H20
condensation reaction
Pathways to other groups:
-
ester + NaOH → sodium salt of acid + alcohol
hydrolysis; saponification
Properties of Carboxylic acids
The carboxyl group is often written in condensed form as –COOH. However, the two oxygen atoms are not bonded to each other. In fact, the carboxyl group consists of a hydroxyl group (–OH) attached to the C atom of a carbonyl group (–C = O).
Carboxylic acids have the properties of acids: a litmus test can distinguish these compounds from other hydrocarbon derivatives. They also react with organic “bases” in neutralization reactions to form organic “salts”.
The melting points of carboxylic acids are higher than those of their corresponding hydrocarbons. This is due to the increased intermolecular attractions of the polar carboxyl functional groups. This explanation is supported by the significantly higher melting points of analogous acids with an abundance of carboxyl groups.
Properties of Esters
The functional group of an ester is similar to the carboxyl group of an acid. What it lacks in comparison to an acid is its –OH group the hydroxyl group is replaced by an –OR group. With the loss of the polar –OH group, esters are less polar, and therefore are less soluble in water, and have lower melting and boiling points than their parent acids. Moreover, the acidity of the carboxylic acids is due to the H atom on their –OH group, and so esters, having no –OH groups, are not acidic.
The low-molecular-mass esters are detected by scent because they are gases at room temperature. The larger, heavier esters more commonly occur as waxy solids.
Section 1.8 AMINES and AMIDES
Amine: An ammonia molecule in which one or more H atoms are substituted by alkyl or aromatic groups.
Amide: An organic compound characterized by the presence of a carbonyl functional group (C = O) bonded to a nitrogen atom.
Functional groups:
Preparation:
RX + NH3 → amine + HX
RX + R2NH → amine + HX
Reactions:
amine + carboxylic acid → amide + H2O (condensation reaction)
amide + H2O → carboxylic acid + amine (hydrolysis reaction)
Properties of Amines
Amines have higher boiling and melting points than hydrocarbons of similar size, and the smaller amines are readily soluble in water. This can be explained by two types of polar bonds in amines: the N – C bonds and any N – H bonds. These bonds are polar because N is more electronegative than either C or H. These polar bonds increase intermolecular forces of attraction, and therefore, higher temperatures are required to melt or to vaporize amines.
Where N – H bonds are present, hydrogen bonding also occurs with water molecules, accounting for the high solubility of amines in water. Since N – H bonds are less polar than O – H bonds, amines boil at lower temperatures than alcohols at similar size.
Properties of Amides
Amides are weak bases, and are generally insoluble in water. However, the lower-molecular-weight amides are slightly soluble in water because of the hydrogen bonding taking place between the amides’ polar N – H bonds and water molecules.
Amides whose N atoms are bonded to two H atoms have higher melting points and boiling points than amides that have more attached alkyl groups, this can also be explained by increased hydrogen bonding, requiring higher temperatures for vaporization.