Alkanes, alkenes and Alkynes - Chemistry 1 - Introduction |, Study notes of Chemistry

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FHCHS

Chapters 10 & 11

Part II

Alkanes, Alkenes

and Alkynes

FHCHS

Module 2

FHCHS Using the prefix, infix, and suffix to help identify compounds.

Alkanes

FHCHS Physical Properties of Alkanes.

• (^) The higher the molecular weight, the stronger the London dispersion forces (and hence the intermolecular attraction ).
• (^) Stronger intermolecular force results in higher bp and mp.

Alkanes

FHCHS Physical Properties of Alkanes.

• (^) Consider the data shown below.
• (^) Increased branching results in lower bp and mp. Why?
• (^) Increasing the degree of branching decreases the surface area of the molecule. This decreases the London dispersion forces (and hence the intermolecular attraction).

Alkanes

FHCHS Reactions of Alkanes

1. Combustion: alkanes burn readily in air.
2. Halogenation
3. Cracking (Pyrolysis)

Alkanes

4 2 2 2 2 2

Methane

CH  O  CO  H O 3 2 3 2 2 2

P

r

CH CH CH O C

a

O H O

op ne

3 3 2 3 2

Ethane

+ Cl + HCl

Chloroethane

CH CH light CH CH Cl

0 0 2 Alkane 400  600 C H + smaller alkanes + alkenes

FHCHS Conformations (3-D Shapes) of Alkanes

• (^) Multiple 3-D shapes are possible due to rotation about the C-C single bond. Here are three possible conformations for butane:

Alkanes

FHCHS Classes Continued

• (^) (3) Arenes: Contain one or more benzene rings. Example: Shape and Structure of Alkenes
• (^) Each double-bonded carbon of an alkene is attached to three other groups that lie in the same plane.
• (^) Each bond angle  120 o.
• (^) Unlike a carbon-carbon single bond, a carbon-carbon double bond does not allow free rotation.

Alkenes and Alkynes

FHCHS Shape and Structure Continued

• (^) Because there is no free rotation about a carbon-carbon double bond, an alkene such as 2-butene may exist as two distinct forms or isomers.
• (^) The isomer with the hydrogens on the same side of the double bond is called the cis isomer.

Alkenes and Alkynes

FHCHS IUPAC Nomenclature for Alkenes

• (^) The parent chain is the longest chain that includes the double bond.
• (^) Number the parent chain to give the double bonds the lowest number.
• (^) Each double bond is identified by specifying only one of the two carbons.

Alkenes and Alkynes

FHCHS IUPAC Nomenclature for Alkenes

• (^) In a cyclic compound, a number is not needed to denote the position of the functional group because the ring is always numbered so that the double bond is between carbons 1 and 2. Alkenes that contain multiple double bonds
• (^) Dienes: Alkenes that contain two double bonds.
• (^) Trienes: Alkenes that contain three double bonds.
• (^) Polyenes: Alkenes that contain “many” double bonds.

Alkenes and Alkynes

FHCHS Problem

• (^) Name the following alkenes:
• (^) Solution

Alkenes and Alkynes

FHCHS IUPAC Nomenclature for Alkynes

• (^) Change infix from “-an-” to “-yn-.”
• (^) The parent chain is the longest chain that includes the triple bond.
• (^) Number the parent chain to give the triple bonds the lowest number.

Alkenes and Alkynes

FHCHS Solution to Problem 1

Solution to Problem 2

Alkenes and Alkynes

FHCHS Problem

• (^) Draw a structural formula for each of the following compounds.
• (^) (a) 2,3-dimethyl-2-pentene, (b) 3-ethyl-3-methyl-1-pentyne,
• (^) (c) 2-methyl-1-butene Solution (a) We know that the parent chain has 5 carbons with a double bond at C2. So we draw and number the parent chain (omit H for now) as shown below. There are 2 methyl (CH 3 -) substituents, one on C2 and the other on C3. Adding these to the parent chain gives:

Alkenes and Alkynes

1 2 3 4 5

C  C  C  C  C

1 2 3 4 5

C C C C C

C C