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Chemistry 11 – Alkenes and Alkynes Notes

As you now know, alkanes are known as saturated hydrocarbons because they contain single-bonded carbon atoms with the maximum number of hydrogens. However, there are hydrocarbons that contain multiple-bonded carbon atoms. These are known as unsaturated hydrocarbons because they have less hydrogen atoms when compared to equivalent alkanes. We will investigate two types in this section: alkenes and alkynes. Alkenes – Double-Bonded Carbon Atoms

• Alkenes: an organic compound containing a carbon-carbon double bond

• Double bonds can be found at the end or in the middle of the carbon chain

• Alkene structure can be represented in two ways:

H CH3 \ /

C = C or CH2 = CH – CH3 / \ H H Naming Alkenes Rules: The rules are similar to the Branched Alkanes except for the following:

1. Identify the longest continuous carbon chain that contains the double bond.

2. Change the ending for the parent chain to “en e”

3. Number the parent chain so that the double bond gets the lowest possible number.

4. Use a number followed by a dash in front of the name of the parent chain to indicate which carbon the double bond is bonded to. (eg. 2-butene)

Example: CH2 = CH – CH2 – CH3 " Double bond found between C1 and C2 of butene 1 2 3 4 1-butene

Alkane, Cyclo alkane , Alkyl halide

C - C

Unsaturated HC

Alkene alkyne④

Hydrocarbon with C=C double bond.

end middle

↳ Must use # to indicate Where ECB.

Each edge or corner

Hi-

H= C atom

C HzC=CHCH3H

, r ' H

µ-

C =Cut,

OI HzCCHCH3 OI

Full structurecondensed Skeletal

.

ENE

↳ # for the side chain is NIT prioritized .

- 4C

⑦③3 4

④aTA 1 - butene

-

not 1,2 - butene

Ctb- CHICH -Ctb2 - butene

CH3- CHZ- C#CHZ3 -Mene

4 3 2 I 1 - butene

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Example:

H2C

HC H

CCH3

CH3 3-methyl-1-butene Geometric Isomers in Alkenes – The Cis / Trans Thing When we were looking at alkanes, we found out that all the carbons were single-bonded. This would allow for lots of flexibility in the bond and free rotation around each bond.

The arrows show the free rotation around the carbon atoms in propane.

However, if we look at alkenes, they contain a double bond in the carbon backbone. What does that do to the geometry of those carbon atoms directly affected by the double bond?

② 34 4C⑦

④¢ox3 - methyl

-1- butene

Location for c=C must

3- methylhave smallest #

,not side chain

.

notetrahedral

to

109.50

1200

T ,

trigonal X

planar

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• The tetrahedral shape of the carbon atom now becomes trigonal planar!

• The bond angle between the three atoms attached to the double-bonded carbon is 120°

• The double bond provides more strength to the molecule. It prevents the affected carbon atoms from twisting around the double bond.

The rigid structure of alkene carbons has an important consequence: Geometric Isomers!

• Two requirements for cis – trans isomers are:

1. A double bond 2. A hydrogen attached to each carbon involved in a double bond

Example: 2-butene has two isomers possible H3C CH3 H3C H \ / \ / C = C C = C / \ / \ H H H CH3 The two hydrogen atoms are The two hydrogen atoms are on the same side of the on opposite sides of the double bond. double bond. Cis – 2 – butene Trans – 2 – butene Alkynes – Triple-Bonded Carbon Atoms

• Alkynes: an organic compound containing a carbon-carbon triple bond

• Triple bonds can be found at the end or in the middle of the carbon chain

• Alkyne structure can be represented in two ways:

CH3 – C ≡ C – CH3 or CH3C ≡ CCH3

trigonal planar

1200

prevents

twisting

geometric isomers.

( cis-trans )geometric

and

⑤ Hand4C

2- - butene

Step DDraw a horizontalline over EC

step ②Are there 2ft ?

CTS - 2 - butene one H on each trans - 2 - butene

⑤ Dc in e=c

Hydrocarbons w/ CEC triple bond

end middle

↳ must indicate Where CEC is using #.

condensed-

H H OR

I ②③14HH - ¢ - CE C - C

,

-

skeletal

H H

4C

2-

butyne

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Naming Alkynes Rules: The rules are similar to the Alkenes except for the following:

1. Identify the longest continuous carbon chain that contains the triple bond.

2. Change the ending for the parent chain to “y ne” Example: 1 2 3 4 5

HC ≡ C – CH2 – CH – CH3 " 1-pentyne parent chain | CH3 " 4-methyl side chain 4-methyl-1-pentyne Example:

CH2

CHC

C HC

CH3

CH3

Br

H3C

2-bromo-5-methyl-3-heptyne

Geometry of Alkynes Alkynes contain a triple bond in the carbon backbone which is very rigid. It causes the carbons affected to be “locked” to each other forming straight-line structures.

• The tetrahedral shape of the carbon atom now becomes linear!

• The bond angle between the two atoms attached to the triple-bonded carbon is 180°

• The triple bond provides super strength to the molecule. It locks the affected carbon atoms and prevents them from twisting around the triple bond.

• Geometric isomer does not exist for alkynes.

YNE

5C⑦②3 4 5

I - pentyne } 4- methyl - I - pentyne4- methyl

← 5- methyl7 Dc

6 5 ④ 2 - bromo - 5- methyl -③ I3- heptyne

2

a 2 - bromo

←Linear

¥800linear

1800

locks

prevents twisting

- alkynes . Only some alkenes have-

cis I trans isomers.

HF : 023,25 , try 027

HI # 19 all naming . Drawing in class next time