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Hydrocarbons
Functional Groups
CHEMISTRY 11
MS. MCGRATH
Functional Groups
• A functional group is a reactive group of bonded atoms that
appears in all the members of a chemical family.
• The functional group is the reactive part of an organic molecule.
Each functional group behaves and reacts in a characteristic way.
Thus, functional groups help to determine the physical and chemical
properties of compounds.
• For example, the reactive double bond is the functional group for
an alkene.
Functional Groups
Common Functional Groups – see page 378
• Note the functional groups
• Organic compounds are named according to their functional group
• We have seen alkanes, alkenes, alkynes, cyclic hydrocarbons and
aromatics (benzene)
• We are going to look at alcohols, aldehydes, ketones, ether
Functional Groups
1. Compounds with the same functional group often have similar
physical properties
2. Compounds with the same functional group react chemically in
very similar ways
Functional Groups
• Functional groups usually contain polar bonds
• Examples of these bonds are O-H, C-O, C=O
• Many hydrocarbons contain C-H bonds which are weak dipoles.
The several C-H bonds partially cancel each other out. There C-H
bonds contribute very little to the polarity of a hydrocarbon – they
are neglected when considering the polarity of the molecule
Physical Properties
Substituted Hydrocarbons
Recall from Chapter 6, we learned about 3 intermolecular forces:
1. Hydrogen Bonds
2. Dipole Interactions
3. Dispersion Forces
** Note melting points of organic compounds follow approximately
the same trend as do their boiling points **
Physical Properties
Hydrogen Bonds
• Molecules that O-H, N-H and F-H bonds, they can form hydrogen
bonds with themselves and water
• These molecules have higher boiling points than similar molecules
that cannot form hydrogen bonds
• For example, alcohols that have an O-H bond can form hydrogen
bonds but alkanes cannot
• Shorter chained molecules that can form hydrogen bonds are
soluble in water
Physical Properties
Dipole Interactions
• Polar molecules usually have a higher boiling point than non-polar
molecules of a similar size but not as high as molecules than can
form hydrogen bonds
• For example, ethanol, CH3CH2OH, is polar and can form hydrogen
bonds. Methoxymethane, CH3OCH3, is also polar but cannot form
hydrogen bonds. Ethane, CH3CH3, is non-polar and cannot form
hydrogen bonds. Ethanol has the highest boiling point, then
methoxymethane and ethane has the lowest.
Physical Properties
Dispersion Forces
• These are weak forces
• A molecule with a greater number of carbon atoms usually has a
higher boiling point than the same type of molecule with fewer
carbon atoms
• For example, hexane, CH3CH2CH2CH2CH2CH3, has a higher boiling
point than ethane, CH3CH3
Physical Properties
Complete ThoughtLab – Comparing Intermolecular Forces
Page 380
Functional Group
Alcohol
General Formula
R-OH
Where:
R represents an alkyl group
OH represents an hydroxyl group (the functional group of the alcohol
family, written as –OH)
Alcohol