01 / 21

Functional Groups
Identification & Properties

Master the key functional groups that determine the chemical behavior of organic molecules: how to recognize them, name them, and predict their reactivity and physical properties.

Organic Chemistry 1

What Is a Functional Group?

A functional group is a specific arrangement of atoms within a molecule that determines the molecule's chemical reactivity and physical properties. The carbon-hydrogen backbone is relatively inert: it is the functional group that makes organic chemistry interesting.

Why Functional Groups Matter

Reactivity
They determine which reactions a molecule undergoes: substitution, elimination, addition, or oxidation.
Physical Properties
Boiling point, solubility, and acidity are all controlled by the functional group's polarity and hydrogen-bonding ability.
Nomenclature
IUPAC names are built around functional groups: the suffix ("-ol", "-al", "-one") tells you the group instantly.

The Major Functional Groups

Hydroxyl
\(\text{–OH}\)
 Alcohols Carbonyl
\(\text{C=O}\)
 Aldehydes / Ketones Carboxyl
\(\text{–COOH}\)
 Carboxylic Acids Amino
\(\text{–NH}_2\)
 Amines Ester
\(\text{R–COOR'}\)
 Esters Ether
\(\text{R–O–R'}\)
 Ethers Thiol
\(\text{–SH}\)
 Thiols Amide
\(\text{–CONH}_2\)
 Amides Alkene
\(\text{C=C}\)
 Alkenes Alkyne
\(\text{C}\equiv\text{C}\)
 Alkynes Halide
\(\text{–X}\)
 Alkyl Halides
\(\text{X = F, Cl, Br, I}\)
\(\text{R, R' = carbon chains (alkyl groups)}\)

The Hydroxyl Group: Alcohols

\(R\)
\(C\)
\(O\)
\(H\)
 Hydroxyl Group lone pairs Example Ethanol
\(\text{CH}_3\text{CH}_2\text{OH}\)
suffix: \(\text{-ol}\)

Carbonyl Group: Aldehydes vs Ketones

Aldehyde
\(R\)
\(C\)
\(O\)
\(H\)
Terminal \(\text{C=O}\)
suffix: \(\text{-al}\)
 Ketone
\(R\)
\(C\)
\(O\)
\(R'\)
Internal \(\text{C=O}\)
suffix: \(\text{-one}\)

The Carboxyl Group: Acids

\(R\)
\(C\)
\(O\)
\(O\)
\(H\)
\(\text{–COOH}\)
 Acidic H
suffix: \(\text{-oic acid}\)

The Amino Group: Amines

Primary Amine
\( \text{R–NH}_2 \): nitrogen bonded to one carbon and two hydrogens. suffix: -amine
Secondary Amine
\( \text{R}_2\text{NH} \): nitrogen bonded to two carbons and one hydrogen.
Tertiary Amine
\( \text{R}_3\text{N} \): nitrogen bonded to three carbons, no hydrogens on N.

Carboxyl Derivatives Compared

Feature Carboxylic Acid Ester Amide
General Formula \(\text{R–COOH}\) \(\text{R–COOR'}\) \(\text{R–CONH}_2\)
Suffix -oic acid -oate -amide
H-bonding Strong (donor + acceptor) Acceptor only Strong (donor + acceptor)
Boiling Point High Lower Highest
Acidity Acidic Neutral Neutral / weakly basic

Polarity and Hydrogen Bonding

The boiling point and water solubility of organic compounds depend on whether the functional group can form hydrogen bonds. Here is the general ranking:

1
Amides (\(\text{–CONH}_2\)): highest boiling points due to very strong H-bonding via \(\text{N–H}\) and \(\text{C=O}\).
2
Carboxylic acids (\(\text{–COOH}\)): high boiling points, because they form dimers via two hydrogen bonds.
3
Alcohols (\(\text{–OH}\)) and amines (\(\text{–NH}_2\)): moderate H-bonding, lower b.p. than acids.
4
Aldehydes, ketones, esters: dipole-dipole only (no H on O or N to donate). No self-H-bonding.
5
Ethers, alkyl halides, alkanes: weakest intermolecular forces, lowest boiling points.

How to Identify a Functional Group

1
Find the heteroatoms. Look for any atom that is not \(\text{C}\) or \(\text{H}\): \(\text{O}\), \(\text{N}\), \(\text{S}\), or halogens (\(\text{F, Cl, Br, I}\)).
2
Identify double/triple bonds. \(\text{C=C}\) (alkene), \(\text{C}\equiv\text{C}\) (alkyne), \(\text{C=O}\) (carbonyl).
3
Check what is attached to \(\text{C=O}\). \(\text{–H}\) = aldehyde, \(\text{–R}\) = ketone, \(\text{–OH}\) = carboxylic acid, \(\text{–OR'}\) = ester, \(\text{–NH}_2\) = amide.
4
Check single-bonded O or N. \(\text{–OH}\) (no \(\text{C=O}\) adjacent) = alcohol, \(\text{R–O–R'}\) = ether, \(\text{–NH}_2\) (no \(\text{C=O}\)) = amine.
5
Rank by priority. If multiple groups exist, the highest priority group determines the suffix in the IUPAC name (carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine).

Example 1: Identify the Groups

Molecule: \( \text{HOCH}_2\text{CH}_2\text{CHO} \) (3-hydroxypropanal)

\(O\)
\(H\)
\(C\)
\(\text{H}_2\)
\(C\)
\(\text{H}_2\)
\(C\)
\(O\)
\(H\)
Alcohol (\(\text{–OH}\))
Aldehyde (\(\text{–CHO}\))

Example 2: Boiling Point Prediction

Problem: Rank the following three-carbon compounds by boiling point (lowest to highest):

Propane
\( \text{CH}_3\text{CH}_2\text{CH}_3 \)
No functional group (alkane)
Propanal
\( \text{CH}_3\text{CH}_2\text{CHO} \)
Aldehyde (\(\text{C=O}\), dipole only)
Propanol
\( \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \)
Alcohol (\(\text{–OH}\), H-bonds)

Example 3: Predicting Solubility

Problem: Which compound is more soluble in water: butanol (\( \text{C}_4\text{H}_9\text{OH} \)) or diethyl ether (\( \text{C}_2\text{H}_5\text{OC}_2\text{H}_5 \))?

1
Identify the groups: Butanol has \(\text{–OH}\) (alcohol). Diethyl ether has \(\text{C–O–C}\) (ether).
2
Check H-bonding with water: Butanol can both donate and accept H-bonds. Ether can only accept (no \(\text{O–H}\)).
3
Compare carbon chain: Both have 4 carbons, so the hydrophobic portion is similar.
4
Conclusion: Butanol is more soluble because its \(\text{–OH}\) enables stronger interactions with water.

Common Mistakes to Avoid

✕ Confusing Aldehyde and Ketone
Both contain \(\text{C=O}\), but an aldehyde has the carbonyl at the terminal carbon (bonded to H), while a ketone has it internal (bonded to two carbons).
✕ Calling Esters "Acids"
An ester (\(\text{–COOR'}\)) looks like an acid (\(\text{–COOH}\)), but the H is replaced by an R group. Esters are neutral, not acidic.
✕ Ignoring Priority
When naming compounds with multiple groups, students often choose the wrong suffix. Always use the highest-priority group for the suffix.

Summary Table

Group Formula Suffix H-Bond? Acidic/Basic?
Alcohol \(\text{–OH}\) -ol Yes (D+A) Weakly acidic
Aldehyde \(\text{–CHO}\) -al Acceptor Neutral
Ketone \(\text{RCOR'}\) -one Acceptor Neutral
Carboxylic Acid \(\text{–COOH}\) -oic acid Yes (D+A) Acidic
Ester \(\text{–COOR'}\) -oate Acceptor Neutral
Amine \(\text{–NH}_2\) -amine Yes (D+A) Basic
Ether \(\text{R–O–R'}\) ether Acceptor Neutral
Amide \(\text{–CONH}_2\) -amide Yes (D+A) Neutral

Question 1 of 5

Which functional group is present in the compound \( \text{CH}_3\text{COCH}_3 \)?

Question 2 of 5

Which compound would have the highest boiling point?

Question 3 of 5

The IUPAC suffix "-oate" indicates which functional group?

Question 4 of 5

Amines are generally basic because:

Question 5 of 5

A molecule contains both a \(\text{–COOH}\) group and an \(\text{–NH}_2\) group. What is the highest-priority functional group for naming purposes?