McMurry Chapter 3 - PowerPoint Presentation

Slide1

McMurry Chapter 3Organic Compounds: Alkanes and Their Stereochemistry

Organic Chemistry IS. ImbriglioSlide2

Functional Groups

A functional group is a group of atoms within a molecule that has a ___________________________.

The chemistry of every organic molecule, regardless of size and complexity, is determined by the

______________________________

it contains.Slide3

Functional Groups

Molecules are classified by the functional group(s) they contain.

Akenes

:

contain carbon-carbon

________________

bonds

Alkynes: contain carbon-carbon ______________ bondsArenes: contain alternating double and single bonds in a six- membered ring of carbon atomsSlide4

Functional Groups

These polar C-X bonds each exhibit unique but similar reactivityX = Cl, Br, I, O, N, SSlide5

Functional Groups

All ________ ____________ contain a highly polarized C=O double bond and exhibit similar reactivitySlide6

Alkanes

Alkanes , also called aliphatic compounds, are defined as molecules that ______________________________.

Alkanes are also referred to as saturated hydrocarbons.

Saturated

– containing only single bonds; no pi bonds or rings

Hydrocarbon

– made up of only carbon and hydrogenSlide7

Alkanes

All saturated hydrocarbons have the molecular formulaCnH2n+2

Alkanes with carbons connected in a row are called

_________________________________

alkanes.

Straight-chain

alkanes

are named based on the number of carbon atoms they contain:n = 1 CH4 methane n = 7 C

7

H

16

heptane

n = 2 C

2

H

6

ethane n = 8 C

8

H

18

octane

n = 3 C

3

H

8

propane n = 9 C

9

H

20

nonane

n = 4 C

4

H

10

butane n = 10 C

10

H

22

decane

n = 5 C

5

H

12

pentane n = 11 C

11

H

24

undecane

n = 6 C

6

H

14

hexane n = 12 C

12

H

26

dodecaneSlide8

Alkanes

When there are more than three carbons in an alkane, it is possible to have normal and branched-chain isomers.

________________________

:

molecules that have the same molecular formulas but the atoms are connected in a different orderSlide9

Constitutional Isomers

There are two different constitutional isomers of butane and three different constitutional isomers of pentane.Slide10

Draw all possible constitutional isomers of hexane. How many are there?Slide11

Alkanes

Alkyl GroupsWhen an ____________________________ is part of a larger compound, it is referred to as an

alkyl group

.

Alkyl groups are named by removing the –ane

suffix from the parent alkane and replacing it with –

yl

.Slide12

Alkyl Groups

There are two different propyl groups because there are two different types of ______________ hydrogens that can be removed from propane.Slide13

How many different butyl groups can you draw (each with the formula C4

H9)?Slide14

Alkyl Groups

_____________________________________________One way to distinguish different alkyl groups is by the number of other carbon atoms attached to the branching carbon.Slide15

Primary (1

), Secondary (2), and Tertiary (3)

Used to classify molecules and distinguish between different sites within a moleculeSlide16

Alkyl Groups

1°, 2°, and 3° are used to differentiate between the four butyl isomers.But there are eight different pentyl groups and seventeen hexyl groups.We need a more systematic way of naming alkanes and alkyl groups.

Butyl Groups – Common NamesSlide17

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) developed a systematic way of naming organic molecules.

Every IUPAC name uses the parent alkane backbone as the base name.Slide18

IUPAC Nomenclature

We’ll use this molecule as an example as we walk through the naming rules…Slide19

IUPAC Nomenclature

Step 1: Find the longest carbon chain. If there are two different chains of equal length, choose the one with the larger number of branch points as the parent chain.

Step 2:

Number the carbon chain, beginning with the end closest to the nearest branch point.

If there is branching an equal distance away from both ends, begin numbering at the end nearer the second branch point. The first point of difference breaks a tie.Slide20

IUPAC Nomenclature

Step 3: Identify and number the substituents

.

Use prefixes (

di-, tri-, tetra-, etc.) if there is more than one of the same kind of substituent on the chain.

Step 4:

Write the name as a single word, listing the

substituents

alphabetically before the parent alkane.Use commas to separate numbers and hyphens to separate numbers from words.Do not alphabetize multiplier prefixes (di, tri, tetra, etc

.).Slide21

IUPAC Nomenclature

Complex Substituents

Use IUPAC rules to name branched substituents.

Begin numbering the longest carbon chain at the point of attachment.

Alphabetize complex substituent by the first letter of the complete name (including numerical prefixes).

Put name of alkyl group in parentheses.Slide22

IUPAC Nomenclature

Common NamesSome branched-chain alkyl groups are often named using their common names.You should be familiar with these names and structures.

-

Iso

and neo are the only prefixes that are not hyphenated and are used when alphabetizing.Slide23

Provide IUPAC names for the following compounds.Slide24

Provide structures for the following IUPAC names.4-

t-butylheptane3,4,5-trimethyl-4-n

-propyloctaneSlide25

Properties of Alkanes

Alkanes are fairly inert, but they will undergo

_________________________________________

.

Combustion:

CH

4

+ 2 O

2

 CO

2

+ 2 H

2

O H = -890 kJ/mol

Halogenation:Slide26

Properties of Alkanes

Alkanes are __________ and tend to be ___________ in and

__________

with polar solvents.The melting and boiling points of alkanes increase with increasing size – due to increased London dispersion forces.Slide27

Conformational Analysis of Alkanes

_____________________: study of the three-dimensional arrangement of atoms in molecules

_____________________

:

study of the energetics of different molecular conformations

Conformations:

structures related by bond rotations; usually interconvertible at room temperature

Conformer: short for conformational isomer; term used to indicate a specific conformationSlide28

Conformational Analysis of Alkanes

There is free rotation around C-C sigma bonds.

Different conformations have different energies (different levels of stability).Slide29

Conformational Analysis of Alkanes

_________________________Newman projections are conformational drawings that allow you to look straight down a bond.Very useful for comparing conformersSlide30

Conformational Analysis of Ethane

Staggered

= 60



E

rel

= 0 kJ/mol

E

rel = 0 kcal/molEclipsed = 0



E

rel

= ~12 kJ/mol

E

rel

= 3.0 kcal/mol

Skewed

0 < 

< 60



0 < E

rel

< 12 kJ/mol

0 < E

rel

< 3.0 kcal/molSlide31

Draw Newman projections for the eclipsed and staggered conformers of ethane.Slide32

Rotational Energy Diagram for Ethane

Barrier to Rotation = Ehighest – Elowest

Ethane: Barrier = 3.0 – 0 = 3.0 kcal/mol

(3.0 kcal/mol)Slide33

Conformational Analysis of Ethane

The eclipsed conformer of ethane is higher in energy than the staggered conformer because of torsional strain._______________________________strain caused by electron repulsion between eclipsed bonds

As the dihedral angle gets smaller, the bonds are forced closer together, and the repulsion is increased.

The angle between the bonds () i

s at a maximum in in the low energy staggered conformer (60



) and at a minimum in the high energy eclipsed conformer (0



).Slide34

Conformational Analysis of Ethane

The conformational energies of ethane can be broken down into individual eclipsing interactions.Three pairs of eclipsed

hydrogen bonds lead to

3.0 kcal/mol of torsional

strain.

One H/H-eclipsing

interaction = 1.0 kcal/molSlide35

Draw a Newman projection of propane from the perspective indicated.Slide36

Conformational Analysis of Propane

Staggered = 60



E

rel

= 0 kJ/mol

E

rel

= 0 kcal/molEclipsed = 0 

Erel = ~14 kJ/mol

Erel = 3.4 kcal/molSlide37

Conformational Analysis of Propane

The conformational energies of propane can be broken down into individual eclipsing interactions.Two pairs of eclipsed

hydrogen bonds lead to

2.0 kcal/mol of torsional

strain.

One H/CH

3

-eclipsing

interaction = 1.4 kcal/molSlide38

Draw a Newman projection of the C2-C3 bond of butane from the perspective shown.Slide39

There are two energetically different staggered conformations of butane with respect to the C2-C3 bond. Draw a Newman projection for each and predict which one is lower in energy.Slide40

There are two energetically different eclipsed conformations of butane with respect to the C2-C3 bond. Draw a Newman projection for each and predict which one is higher in energy.Slide41

Conformational Analysis of Butane

To distinguish between the four different conformers of butane, Me/Me is defined as the dihedral angle between the two methyl groups.Slide42

Conformational Analysis of Butane

Butane: Barrier = 4.5 – 0 = 4.5 kcal/mol