AP Chemistry Chapter 22: Organic and Biological Molecules
Group 4A elements, most especially carbon and silicon, form the basis of most natural substances. Silicon
generally forms chains and ring structures with oxygen, which allows it to make up most of Earth‟s rocks, sand and
soil. Carbon, on the other hand, also forms chains and rings, but generally with itself and many other nonmetals.
Because of this, carbon is the basic structural unit for many biomolecules, or life molecules.
22.1 Alkanes: Saturated Hydrocarbons
Organic chemistry is the chemistry of carbon compounds; the chemistry of life molecules.
 Several million carbon compounds are currently known, with more being discovered each day. The reason
for this is that carbon can form four covalent bonds with other atoms.
 Carbon can form single, double, and triple bonds.
 Hydrocarbon- a compound containing only hydrogen and carbon Ex. CH4, C2H6
o Saturated hydrocarbon- a hydrocarbon with no double or triple bonds
 It is saturated with hydrogen!
o Unsaturated hydrocarbon- a hydrocarbon that contains double and/or triple bonds
 It is not “saturated” with hydrogen
Alkanes
 The sinplest type of hydrocarbon is called an alkane.
o Alkanes are composed on carbon atoms single bonded to other carbon atoms or other nonmetals.
o General formula: CnH2n+2
o Alkane names end in –ane
o Alkanes can be written as structural formulas or as condensed formulas
o Alkane melting and boiling points increase with the number of carbons
Question: Why would the melting and boiling points of alkanes increase as the number of carbon atoms increase?
Alkanes with four or more carbons may form isomers, or molecules that have the same atoms bonded in different
ways.
 Straight-chained hydrocarbon- alkane in which carbon forms long “strings” or chains
o Ex. CH3CH2CH2CH2CH2CH3
o C6H14

Branched hydrocarbon- alkane in which carbon chains may be attached to the main
chain
o
CH3
CH3CH2CHCH2CH3
Practice: Draw all of the structural isomers of hexane, C6H14, in the space below.
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Alkanes may also form stereoisomers. Stereoisomers include cis-trans isomers (discussed later) and optical
isomers.
o Optical isomers, or enantiomers, are named due to their ability, in crystal form, to rotate planepolarized light.
 Those that rotated the light to the right were called
dextrorotatory, or right-hand isomers.
 Those that rotated the light to the left were called levorotatory,
or left-handed isomers.
 These two types of crystals were found to be mirror images of
one another; this is described with the word “chiral”.
 Chiral molecules exhibit different chemical affects when used
in biological systems. Just as you shake hands with the right hand of someone if you are
using your right hand, a biomolecule may only bind with one version of a chiral molecule
and not another.
Hydrocarbon derivative- a compound with some hydrogen atoms replaced by other elements (O, N, F, Cl,
Br, I)
Ex. CH3Br
Cyclic Alkanes – alkanes in which the carbon atoms have formed rings
o The general formula for a cyclic alkane is CnH2n
o The simplest cyclic alkane is C3H6, or cyclopropane
Alkyl Groups – If one alkane is attached as a side chain on a longer-chain
hydrocarbon, it is called an alkyl group. Alkyl groups have the same name as
the alkane, but the –ane ending is changed to –yl.
o Ex. Methane 
methyl
o CH4 
-CH3
22.2 Alkenes and Alkynes
 Alkenes- unsaturated hydrocarbons that contain at least one double bond
o General formula: CnH2n
o The carbon atoms of alkenes are held in place around the double bond by sigma as well as pi bonds.
Since this involves the overlap of p orbitals, the carbons around the double bond are fixed in place
and unable to rotate like an alkene.
o This leads to a specific type of stereoisomer called cis-trans
isomerism.
o Cis means “same side” and trans means “opposite side”
 Practice: Draw both cis-trans isomers of pentane, C5H10, in which the double bond in on carbon 2.
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Alkynes- unsaturated hydrocarbons that contain at least one triple bond
o General formula: CnH2n-2
o The carbon atoms around the triple bond exhibit sp hybridization, giving them a linear structure.
Both alkanes and alkynes can also form ring structures, which we will discuss later.
Nomenclature
 The naming system for organic molecules is slightly different than the naming conventions for other
chemical compunds.
 IUPAC (International Union of Pure and Applied Chemistry)- determines the naming system for
hydrocarbons
 Rules for Naming Alkanes, Akenes and Alkynes:
1. Name the longest carbon chain in the structure. If the chain is cyclic, include the prefix cyclo.
2. Name, in alphabetical order, the kinds of groups which are attached to the chain you named in step 1.
Use the appropriate prefix, such as di-, tri-, tetra-, etc., to indicate how many of each group are present.
3. Number the carbon atoms in the longest chain consecutively from the end of the chain which allows the
attached groups to have the lowest numbers possible. Assign to each group you named in step 2 the
number(s) indicating its position(s) on the main chain.
4. If the compound contains double or triple bonds, name the longest carbon chain containing the carboncarbon double or triple bond. The name of the longest chain ends in –ene for an alkene; in –yne for an
alkyne. The position of the double or triple bond in the carbon chain is indicated by a number before the
name of the chain.
5. When assigning numbers to atoms in the chain, start numbering from the end of the chain closest to the
double or triple bond.
Practice Naming These Compounds
Draw: 3-ethyl-2-hexene
Draw: 4-bromo-2,2dichloro-3,5-diethyl-6fluorooctane
22.3 Aromatic Hydrocarbons
 Aromatic hydrocarbons- unsaturated compounds that contain a hydrocarbon ring
o Sometimes called arenes
 Benzene, C6H6, is an arene.
 Benzene is a flat molecule with resonance.
 Shorthand ways to draw benzene:
 A benzene ring used as a substituent on a hydrocarbon chain is called a phenyl group.
 Ex.
Reactions of Hydrocarbons
No matter what type, all pure hydrocarbons are nonpolar due to a lack of free electrons in the structure. This means
that they are not attracted to water. They tend to be relatively stable in the saturated form, but addition of energy to
the system, like a spark or excess heat, as well as varying the molecule to make it unsaturated, can lead to greater
reactivity in hydrocarbons.
 Hydrocarbons react in four distinct ways: Combustion, substitution, addition and dehydrogenation.
 Reactions of alkanes
o Combustion
 Ex. Butane burns.
o Substitution – occurs primarily when a halogen replaces a hydrogen on a carbon chain
 Ex.
o Dehydrogenation – occurs when hydrogen atoms are removed from a carbon chain, forcing two
carbons atoms to form a double bond or ring.
 Ex.

Reactions of alkenes and alkynes
o Addition – occurs when atoms are added to
the carbons of a double bond, breaking the
double bond and saturating the molecule.
 Ex. Halogenation or hydrogenation

Reactions of aromatic hydrocarbons
o Primarily undergo substitution reactions in
which a hydrogen is replaced with another
atom.
 Ex.
22.5 Hydrocarbon Derivatives
 The vast majority of organic molecules contain
elements in addition to carbon and hydrogen.
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Functional Group- a group of atoms or groups of atoms bound to an organic molecule. It is a chemically
reactive part of an organic molecule
You need to learn these!
Alcohols
 Contain a hydroxyl group; -OH
 Named by replacing the hydrocarbon name ending with –ol.
 Question: Alcohols have much higher boiling points than their hydrocarbon counterparts. Explain.

Alcohols can be described as primary secondary and tertiary. Draw one of each below.
Aldehydes and Ketones
 Contain a carboxyl group;
 Aldehydes are names by adding the suffix –al to the hydrocarbon name, while ketones are named by adding
–one.
 In an aldehyde, the carboxyl group is located at the end of a hydrocarbon chain, while in ketones, it is
located in the middle of the chain.
 They are often created by the oxidation of an alcohol. Primary alcohols yield aldehydes and secondary yield
ketones.
Carboxylic Acids and Esters
 Contain a carboxyl group;
 In a carboxylic acid, the carboxyl group is bound to the end of a hydrocarbon chain and is named by adding
–oic acid to the name.
 In esters, the carboxyl group is found in the middle of the chain and is named according to how many
carbons are on each side of the carboxyl group.
 Esters are formed when an alcohol reacts with carboxylic acid, releasing water. This process is called
esterification.
Amines
 Derivatives of ammonia and have an amine group; -NH2
 Amines are named by adding the suffix –amine to the hydrocarbon name, or by adding the prefix amino- if
the group is included with another functional group.
Ethers
 These molecules contain a lone oxygen with hydrocarbon chains bonded to both ends.
 Each hydrocarbon chain is named, followed with –ether.