12/9/2009 1

Chem 105 Wed 12-9-09

Today: Chapt 10 Carbon Compounds (cont’d)

1) Final Exam Information

2) Optical Isomers – mirror image molecules

3) Alkenes (CnH2n)

4) Saturated and unsaturated fats

5) Aromatic hydrocarbons

6) Petroleum chemistry

7) IAS questionnaire

12/9/2009 2

Final Exam Information

1. Friday, Dec. 18, 1-3 pm (this room)

2. Covers Chap 1-10 Kotz, lecture and lab

3. American Chemical Society “General Chemistry

First Semester”

4. 70 Multiple choice questions (a-d)

5. Extra sheet with several substitute questions

(ideal gas OUT, organic chemistry IN.)

6. Use the ACS book Preparing for Your ACS

Examination in General Chemistry

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Sections Not covered in lecture, OWL, Final Exam

10.5 Polymers

NO molecular (I.U.P.A.C) names of:

Alcohols, aldehydes, ketones, carboxylic acids, or

fatty acids

Do need to name, recognize the functional groups

and draw formulas for specific examples of

alkane, alkene, alkyne, alcohol, ether, aldehyde,

ketone, carboxylic acid, carboxylic amide.

Alkanes and Alkenes, YES

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Optical Isomers

Molecules containing a tetrahedral atom with four different atoms

or groups can exist in “right-handed” and “left-handed” forms.

These have identical physical properties, except how they interact

with polarized light. One isomer twists polarized light beam

clockwise, the other isomer twists it counterclockwise.

Hence, they are “optical isomers” . This type of molecule is “chiral”

(ky’ rəl).

C

X

Y

Z

W

CHClFH3CH3C

H

Cl

F

12/9/2009 5Fclboth.hin

12/9/2009 6

Optical Isomers

“L-phenylalanine” (l) tastes bitter. “D-phenylalanine” (r) tastes sweet.

The “L” isomers of amino acids occur naturally in proteins. “D” isomers are rare.

H

NH2HO2C

H

H2NCO2H

12/9/2009 7

Alkenes contain C=C bond (CnH2n)

Some alkenes can exist in cis- or trans- geometric isomers

H3C

CH3H

H

H3CCH3

H H cis-

trans-

Same atoms or groups

– compare their

positions on C=C.

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Geometric isomers (cis and trans on C=C)

C=C bonds contain π-bond that depends on parallel orientation of the 2 p-

orbitals on bonded carbons. This prevents twisting around the bond. Therefore

C=C bonds containing different atoms can exist as cis- or trans- isomers

C l C l

HH

C l C l

H C H3

cis isomers

No

C l

C l

H

H

C l

C lH

C H3

trans isomers

12/9/2009 9

Naming alkenes

(1) Find longest chain containing BOTH CARBONS of C=C.

(2) Number the carbons starting at the end nearer C=C.

(3) Locate the C=C along the chain using # of FIRST C=.

cis-2-pentene

Location of C=C

Main chain 5 C’sMain chain

contains C=C

H3C CH3

H H

1

23

4 5

pentene

2-pentene

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1

2 3

4 5

6

78

9CH3

CH3

CH3H3C

4-nonenediethyl-4,5-trans-

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Alkanes are referred to as “saturated hydrocarbons ”. They

have the maximum number of H’s for a given # of C’s.

Alkenes are “unsaturated” - they have fewer H’s.

Unsaturated fats contain C=C bonds along the fatty acid

chain. These are usually cis- because the C=C plays a

structural role in lipid bilayers.

12/9/2009 12

Saturated fatty

acyl group

glycerol

phosphate

“Lard” is derived from cell

membranes of animals. It is

rich in saturated fatty acid

components.

The straight chains pack

tightly into a lipid bilayer with

proper biological function at

37°C.

Section of

lipid

bilayer

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Plant and bacterial cell

membranes contain fatty acid

components rich in cis-double

bonds.

These chains are kinked by

the cis bond, resulting in

disordered lipid bilayer that is

more fluid. It remains flexible

at low temperatures, which

maintains the proper biological

function of the cell membrane.

C=C

12/9/2009 14

trans-double bonds in fat

are man-made.

They are created during

partial hydrogenation

reactions that add H2 to

C=C bonds using a metal

catalyst.

The trans-acid packs into

membranes in a manner similar

to saturated fats. This may be the

basis of the unhealthy effects of

fats containing lots of trans fatty

acids.

http://en.wikipedia.org/wiki/Trans_fat

12/9/2009 15

Alkynes contain a C≡C triple bond.

HH

Each sp-hybridized carbon contains

two unhybridized p-orbitals

C C HH

The four p-orbitals combine

to form two π-bonds.

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C CCH3CH2 CH2

CH2 CH

CH3

CH3

1 2 3 4

5

6 7

7-methyl-3-octyne

8

H-C≡C-H common name is “acetylene”

(“ethyne” is the systematic name, but is not used.)

Uses: acetylene is welding gas

larger alkynes used as synthesis intermediates because

the C≡C bond can be modified in many ways.

Some natural molecules such as dynemicin anti-cancer

drug contain C≡C bonds.

12/9/2009 17

Aromatic hydrocarbons

12/9/2009 18

Aromatic hydrocarbons contain one or more benzene rings.

Benzene itself is C6H6. It is a resonance hybrid of two

equivalent forms. This leads to an unusually stable molecule.

This is called an “aromatic ring” which refers to the special

stability of 6 π-electrons in a ring of sp2-atoms.

C

CC

C

CC

H

H

H

H

H

HC

CC

C

CC

H

H

H

H

H

H

Sometimes written this way to

emphasize bonding symmetry:or

12/9/2009 19

H

H H

H

HH

Enhanced stability of the aromatic ring is due to formation of a

low-energy π-molecular orbital extending around the ring.

12/9/2009 20

Toluene (methylbenzene) is 28 kcal/mol more

stable than the next-most stable C7H8 isomer

-1610

-1600

-1590

-1580

-1570

-1560

-1550

-1540

-1530

Bin

din

g e

nerg

y (

kcal/

mo

l)*

* calculated with PM3 semi-empirical method

CH3

CH2

CH3 H2C

H3C CH2

CH2

There are >100 isomers of C7H8!

Toluene

Not

aromatic

Aromatic

12/9/2009 21

H

H

H

H

H

CH3

H

H

H

H

CH3

CH3

H

H

H

CH3

H

CH3

H

HH

H

CH3

CH3

Toluene

(methylbenzene)

ortho-xylenemeta-xylene

para-xylene

12/9/2009 22

Hydrocarbon properties

Boiling points vs molar mass

GC separation

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12/9/2009 24

Gas chromatogram of C1-C6 hydrocarbons

Courtesy of R. Stolzberg

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Courtesy of R. Stolzberg

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Courtesy of R. Stolzberg

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Distillation

Courtesy of R. Stolzberg

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Courtesy of R. Stolzberg

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300-600 oC

"catalysts"

Refinery chemistry:

Reforming gives lots more gasoline

Crude oil Lots of gasoline

12/9/2009 30(from North Pole refinery)

The benzene content of

Alaska’s distilled-only gasoline

is much greater than regular

gas manufactured in Lower 48

refineries.

Courtesy of R. Stolzberg

12/9/2009 31

The End