Functional group: group of atoms with characteristic

chemical behavior

Reactions of organic molecules

are governed by their functional

groups, regardless of size and

complexity of the molecule.

Alkene: contains C=C double bond

Alkyne: contains C≡C triple bond

Arene: contains a benzene

(or other aromatic) ring

Many functional groups have an

electronegative atom bonded to carbon

(this makes carbon δ+)

Chapter 3: Organic Compounds: Alkanes and Their Stereochemistry

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Carbonyl group: C=O double bond

Many functional groups contain a carbonyl:

A few other functional groups:

Functional groups

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Alkanes contain no functional groups - only C-H and C-C

single bonds (all ____ hybridized carbons)

Methane: CH4

Ethane: C2H6

Propane: C3H8

Butane: C4H10

If an alkane has n carbons, how many hydrogens does it

have?

Alkanes are saturated with hydrogen (no more H's can

be added) - a.k.a. aliphatic compounds.

The alkanes shown above are straight-chain or normal

alkanes (C's connected to no more than 2 other C's)

Starting with butane, alkanes can be branched - one or

more C's connect to 3 or 4 other C's) - pencil test

3.2 Alkanes and alkane isomers

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Must have same molecular formula�

Constitutional isomers: molecules that differ in how

their atoms are connected

Constitutional isomers

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The molecular formula for butane contains no structural

information (C4H10)

Structures can be drawn out showing all bonds, but this

is tedious and only used when absolutely necessary...

Condensed structures can efficiently show how the

atoms are connected (although no bonds are drawn)...

Line structures are handy too...

Drawing alkanes

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1-4 are historical names. 5 and up are named based on

the number of carbons. Memorize 1-10.

Common isomer names:

Isobutane:

Isopentane:

Neopentane:

Names of straight-chain (n-) alkanes

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Wildcard R for any alkyl group�

Replace -ane with -yl in name�

Alkyl group: partial structure that remains after

removing one hydrogen from an alkane molecule

Methane (CH4) →

Ethane (CH3CH3) →

Propane (CH3CH2CH3) →

Butane →

Isobutane →

Pentane →

Isopentane →

Neopentane →

3.3 Alkyl groups

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Primary: carbon at end of chain�

Secondary: carbon in the middle of a chain�

Tertiary: carbon attached to 3 other C's�

Alkyl groups are classified by their connection site (where

the hydrogen was removed from)

Primary, secondary, tertiary, and quaternary can be used

to describe the location of functional groups:

Identify the primary, secondary, and tertiary carbons and

hydrogens in the compounds below:

Types of alkyl groups

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Even complex branched organic molecules can be named

using the systematic IUPAC naming process.

If tied, use the chain with the most branches�

Find the parent hydrocarbon chain (longest

continuous chain of carbons - may turn corners!)

1.

If tied, look for the nearest 2nd branch�

Number atoms on the main chain (start with the end

nearest to a branch point)

2.

Identify and number the substituents3.

Combine multiple identical substituents with

multiplier prefixes like di-, tri-, tetra-, etc.

�

Alphabetize with substituent names but not

multiplier prefixes

�

Use hyphens to separate a number and a letter�

Use commas to separate numbers�

Molecule is named as a single word4.

3.4 Naming alkanes

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Branching off sub-structures is handled by naming the

sub-structure as if it were a compound itself:

Complex structures

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Alkanes are sometimes called paraffins because they

have little affinity for reactions with other substances.

Combustion: alkanes react with oxygen to

produce carbon dioxide and water, and release a

large amount of heat

1.

Radical halogenation (which will be detailed in a

later chapter): H's replaced with Cl's in the

presence of strong ultraviolet light (hν)

2.

There are 2 reactions that alkanes will readily undergo:

Alkanes have a predictable trend in boiling and melting

points due to the increase in dispersion forces:

3.5 Properties of alkanes

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Stereochemistry: concerned with the 3-dimensional

aspects of molecules

Because σ bonds exist directly between two atoms,

rotation is possible around C-C bonds in open-chain

molecules.

Conformations: different arrangement of molecules as a

result of bond rotations

Conformers: molecules with different conformations

(conformational isomers)

3.6 Conformations of ethane

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The rotation about the C-C bond in ethane is not

completely free rotation.

Torsional strain is the barrier to rotation that causes

some conformers to be more stable than others.

each H on C1 is 180o across from an H on C2�

this minimizes torsional strain (most stable)�

Staggered conformation: the hydrogens on carbons 1

and 2 are as far away from each other as possible

each H is directly in front of another�

this maximizes torsional strain (least stable)�

Eclipsed conformation: the hydrogens on C1 are as close

as possible to the hydrogens on C2

Torsional strain

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Propane has similar conformations to ethane, except the

torsional strain is slightly larger because one of the H-H

interactions is replaced by an H-CH3 interaction.

Anti conformation: CH3 groups are 180o apart�

Gauche conformation: CH3 groups are 60o apart�

Butane is more complicated because not all staggered

and eclipsed conformations have the same energy.

3.7 Conformations of other alkanes

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The additional interaction in the butane conformations is

steric strain, the repulsive interaction between atoms

trying to occupy the same space.

Butane totally eclipsed conformation (0o):

Steric strain

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