Organic Functional Groups Cheat Sheet

A printable reference covering hydrocarbons, alcohols, carbonyls, carboxylic acids, esters, amines, amides, and functional group properties for grades 11-12.

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Organic functional groups are the reactive parts of carbon compounds that control naming, structure, and chemical behavior. This cheat sheet helps students recognize common groups quickly from skeletal structures, condensed formulas, and reaction patterns. It is useful for comparing families such as alcohols, aldehydes, ketones, acids, esters, amines, and amides. Grade 11-12 chemistry students need these patterns for nomenclature, reactions, spectroscopy, and synthesis problems. The most important idea is that atoms such as $O$ , $N$ , and halogens change polarity, intermolecular forces, boiling points, acidity, and reactivity. Carbonyl compounds contain the $C=O$ group, while alcohols contain $R-OH$ and amines contain $R-NH_2$ , $R_2NH$ , or $R_3N$ . Carboxylic acids, esters, and amides are carbonyl derivatives with different atoms attached to the carbonyl carbon. Recognizing these groups lets students predict reactions such as oxidation, reduction, esterification, hydrolysis, and acid-base behavior.

Key Facts

Alkanes contain only single bonds and follow the acyclic formula $C_nH_{2n+2}$ .
Alkenes contain at least one $C=C$ bond and acyclic monoalkenes follow $C_nH_{2n}$ .
Alkynes contain at least one $C\equiv C$ bond and acyclic monoalkynes follow $C_nH_{2n-2}$ .
An alcohol has the functional group $R-OH$ , while an ether has the functional group $R-O-R'$ .
Aldehydes have the carbonyl group $R-CHO$ , while ketones have the carbonyl group $R-CO-R'$ .
Carboxylic acids have the group $R-COOH$ and are usually more acidic than alcohols because the carboxylate ion $R-COO^-$ is resonance stabilized.
Esters form from a carboxylic acid and an alcohol by $RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O$ .
The index of hydrogen deficiency is $IHD = \frac{2C + 2 + N - H - X}{2}$ , where $X$ is the number of halogen atoms.

Vocabulary

Functional group: A functional group is a specific atom or group of atoms in an organic molecule that gives the molecule characteristic reactions and properties.
Hydrocarbon: A hydrocarbon is an organic compound made only of carbon and hydrogen atoms, such as an alkane, alkene, alkyne, or aromatic compound.
Carbonyl group: A carbonyl group is the polar $C=O$ group found in aldehydes, ketones, carboxylic acids, esters, and amides.
Carboxylic acid: A carboxylic acid is an organic compound containing the $R-COOH$ group, which can donate $H^+$ to form $R-COO^-$ .
Ester: An ester is a carbonyl compound with the group $R-COOR'$ that often forms from a carboxylic acid reacting with an alcohol.
Amine: An amine is an organic base containing nitrogen, commonly written as $R-NH_2$ , $R_2NH$ , or $R_3N$ .

Common Mistakes to Avoid

Confusing aldehydes and ketones is wrong because aldehydes have the carbonyl at the end of a chain as $R-CHO$ , while ketones have it between two carbon groups as $R-CO-R'$ .
Calling every $O$ -containing group an alcohol is wrong because alcohols require $R-OH$ , while ethers are $R-O-R'$ and carbonyl compounds contain $C=O$ .
Forgetting that halogens count like hydrogens in $IHD = \frac{2C + 2 + N - H - X}{2}$ gives the wrong unsaturation value.
Assuming carboxylic acids and alcohols have similar acidity is wrong because $R-COO^-$ is resonance stabilized, but $R-O^-$ is not stabilized in the same way.
Drawing an ester as $R-O-CO-R'$ without checking connectivity can be misleading because the ester functional group is best recognized as $R-COOR'$ with the carbonyl carbon bonded to oxygen.

Practice Questions

1 Calculate the index of hydrogen deficiency for $C_6H_{10}O_2$ using $IHD = \frac{2C + 2 + N - H - X}{2}$ .
2 A compound has formula $C_4H_8O$ and shows a strong carbonyl absorption near $1715\ \mathrm{cm^{-1}}$ . If it does not react like an aldehyde, which functional group is most likely present?
3 Classify each functional group in the molecule $CH_3CH_2COOCH_3$ and name the main family it belongs to.
4 Explain why $CH_3COOH$ is more acidic than $CH_3CH_2OH$ even though both contain an $O-H$ bond.

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