CHEM 494 Lecture 3ramsey1.chem.uic.edu/chem494/page7/files/CHEM 494 Lecture... · 2012. 9. 25. ·...

University of Illinois at ChicagoUICCHEM 494

Special Topics in Chemistry

Prof. Duncan WardropSeptember 24, 2012

CHEM 494 - Lecture 3

UICUniversity of Illinois at Chicago CHEM 494, Fall 2012

SlideLecture 3

Course Website

2

http://www.chem.uic.edu/chem494

SyllabusCourse PoliciesOther handoutsAnnouncements (Course News)Course Calendar



Chemical Properties of Alkanes


SlideLecture 3

Hydrocarbons are Weak Acids(Carbanions are Strong Bases)

4

pKa = 26

pKa = 43

pKa = 45

pKa = 62

Incr

eas

ing

Aci

d S

tre

ngt

h

C CH H C CH + H

C C C C + HH

H

H

H

H

H H

CCCC C

C H

HH

H

H HCC

CC C

C

HH

H

H H

+ H

C CHHH

HH

HC C H

HHH

H+ H


SlideLecture 3

Combustion is Exothermic

5

CH4 + 2O2

CO2 + 2H2O

CH4 + 2O2 ➞ CO2 + 2H2O

ΔHº (enthalpy of rxn)

Ea (activation energy)

ΔHº = Hº(products) - Hº (reactants)

exothermic ΔHº = negative

higher potential energy of reactant hydrocarbon = larger enthalpy of combustion

endothermic ΔHº = positive

higher energy of reactants =larger heat of combustion =

more exothermic -ΔHº (heat of combustion)


SlideLecture 3

Enthalpy of Reaction & Heat of Combustion

6

heat of combustion = -ΔHº = 890 kJ (212.8 kcal)

heat of combustion = –ΔHº

heat of combustion = -ΔHº = 3529 kJ (843.4 kcal)


SlideLecture 3

Heats of Combustion of Unbranched Alkanes

7

increase number of carbon atoms = increased heat of combustion (-ΔHº)


SlideLecture 3

Heats of Combustion of Unbranched Alkanes

8

increased branching of isomers = increased intramolecular VWF = lower energy (more stable) =

decreased heat of combustion (-ΔHº)intramolecular forces:same electronic forces previously described

more nuclear attractions & more intramolecular VWF =

more stable =

lower energy =

smaller heat of combustion


SlideLecture 3

Octane Rating

9

100: iso-octane (2,2,4-trimethylpentane)

0: heptane

The burning qualities (knocking) of gasoline are compared to the burning qualities of iso-octane and heptane mixtures. This does NOT mean that gasoline contains iso-octane and heptane.lower heat of combustion

slower burning

higher heat of combustionfaster burning


SlideLecture 3

Internal-Combustion Engine: Knocking

10

engine-knocking or detonation: spontaneous combustion of the remaining fuel/air mixture left in the combustion chamber after normal combustion burn initiated by spark-plug

pre-ignition: spontaneous combustion of the fuel/air mixture before the spark plug "res

http://www.streetrodstuff.com/Articles/Engine/Detonation/

More highly branched alkanes produce less energy, but burn more effectively in an internal combustion engine by reducing knocking.


SlideLecture 3

Combustion is an Oxidation Reaction

11

CH4 + 2O2 ➞ CO2 + H2O


SlideLecture 3

Definitions of Oxidation and Reduction

12

a. lose electrons (LEO GER)b. gain bonds to oxygenc. lose bonds to Hd. oxidation # increases

a. gain electrons (LEO GER)b. lose bonds to oxygenc. gain bonds to Hd. oxidation # decreases

All de"nitions above describe the amount of electron density centered on an atom. In summary, any process that decreases electron density, whether formal or informal, is termed oxidation. Likewise, any process that increases electron density, whether formal or informal, is termed reduction.

Oxidation(sometimes represented as [O])

Reduction(sometimes represented as [H] or [R])


SlideLecture 3

Determining Oxidation Numbers on Carbon

13

Three Simple Rules:

1. For each bond to an atom less electronegative than carbon (i.e. H) add (-1).

2. For each bond to an atom more electronegative than carbon (i.e. O) add (+1).

3. For each bond to another carbon atom add (+0).

H3CC

OH

H H

H3CC

O

H

H3CC

O

OHoxidation

reduction

oxidation

reduction

C–H: (-1)C–H: +(-1)C–O: +(+1)C–C: +(0)

Ox. #: –1

C–H: (-1)C–O: +(+1)C–O: +(+1)C–C: +(0)

Ox. #: +1

C–O: (+1)C–O: +(+1)C–O: +(+1)C–C: +(0)

Ox. #: +3


SlideLecture 3

Oxidation States of Carbon

14

-3 -2 -1

smaller number (more negative) = more electron density


SlideLecture 3

Oxidation States of Carbon

15

increasing oxidation state of carbon(increasing number of bonds to oxygen)

-3, -3 -3, -1 -3, +1 -3,+3

CH

HCH HH

HCH

HCH HOH

HCH

HCH HO

CH

HCH OHO

each carbon atom can have a different oxidation number



Alkenes and Alkynes: sp2 & sp Hybridization


SlideLecture 3

Alkenes

17


SlideLecture 3

sp2 Hybridization

18

One p-orbital is Reserved–Not Hybridized


SlideLecture 3

Valence Model of Bonding in Ethylene(with Hybridization)

19

2p

2sp2 2sp2 2sp2

2p

2sp2 2sp2 2sp2

C C

H H H HC CH H

HH1s 1s 1s 1s


SlideLecture 3

Orbitals on sp2-Hybridized Carbons

20


SlideLecture 3

Double Bond: 1 Pi-Bond & 1 Sigma-Bond

21

A double bond is formed by two orbital overlaps

1 pi (π) bond: side-to-side overlap of two p-orbitals; 2

π-electrons

1 sigma (σ) bond: head-to-head overlap of two

sp2-orbitals (not shown in "gure on left)


SlideLecture 3

Acetylenes

22


SlideLecture 3

Self Test Question

23

If the carbon atoms in acetylene are sp hybridized, what set of valence orbitals does each carbon atom contain?

A. one 2s, three 2pB. one 2s, two 2p, one spC. two sp, two p, one sp2

D. three sp, one pE. two sp, two p

C CH H


SlideLecture 3

Hybridization in Acetylene

24

Two p-orbitals are Reserved–Not Hybridized


SlideLecture 3

Valence Model of Bonding in Acetylene(with Hybridization)

25

2p

2sp 2sp

2p 2p 2p

2sp 2sp

C C

H H1s 1s

C C HH


SlideLecture 3

Orbitals on sp-Hybridized Carbons

26


SlideLecture 3

Triple Bond: 2 pi-bonds & 1 sigma-bond

27

A triple bond is formed by three orbital overlaps

2 pi (π) bonds: side-to-side overlap of two sets of

p-orbitals; 4 π-electrons

1 sigma (σ) bond: head-to-head overlap of two

sp2-orbitals


SlideLecture 3

Acetylene As a Fuel

28

Moweaqua, IL Mine Disaster 1932


SlideLecture 3

Hybridization and Acidity

29

increased s-character =increased electronegativity of carbon =

electrons closer to the nucleus =stronger acid

pKa = 26

pKa = 45

pKa = 62Incr

eas

ing

Aci

d S

tre

ngt

h

sp

sp2

sp3

C CH H C CH + H

C C C C + HH

H

H

H

H

H H

C CHHH

HH

HC C H

HHH

H+ H


SlideLecture 3

Molecule of the Week...2,4,6-Tribromoanisole The Smell of Unintended Consequences

30

Read more about tribromoanisole...

Paecilomyces variotii

Br

OH

Br

OMePaecilomyces

variotii

2,4,6-Tribromophenol 2,4,6-Tribromoanisole

BrBr Br Br


SlideLecture 3

Self Test Question

31

Which of the following best depicts a π-bond?

A. aB. bC. cD. dE. e

a.

b.

c.

d.

e.


SlideLecture 3

Summary of Bond Types

32

π-bond

C C C C

σ-bond

bond σ-bonds(head-to-head)

π-bonds(side-to-side)

single 1 0double 1 1triple 1 2


SlideLecture 3

Self Test Question

33

Rank the following hydrocarbons in order of increasing acidity.

A. ethane, ethylene, ethyne

B. ethane, ethyne, ethylene

C. ethyne, ethylene, ethane

D. ethyne, ethane, ethylene

E. none of the above

ethane ethylene ethyne

C CH

H H

HC C HHC C

HHHH

HH

four 2sp3 one 2pthree 2sp2

two 2ptwo 2sp


SlideLecture 3

Hybridization and Acidity

34

increased s-character =increased electronegativity of carbon =

electrons closer to the nucleus =stronger acid

pKa = 26

pKa = 45

pKa = 62Incr

eas

ing

Aci

d S

tre

ngt

h

sp

sp2

sp3

C CH H C CH + H

C C C C + HH

H

H

H

H

H H

C CHHH

HH

HC C H

HHH

H+ H



Conformational Isomers of Alkanes


SlideLecture 3

Isomer Classification

36


SlideLecture 3

Classification of Isomers

37

stereoisomers

Can the molecules be interconverted by

rotation around single bonds?

noyes

conformational con$gurationalWe will continue the

isomer tree in Chapters 5 & 7 where we’ll

encounter subdivisions of con"gurational

isomers

HH

Me

Me

H

H

H

Me

H

Me

HH

anti butane gauche butane


SlideLecture 3

Model Activity

38

1. Make a model of butane.

2. Make a separate model of isobutane.

3. Using a minimum number of changes, convert your model of isobutane into butane.


SlideLecture 3

Self Test Question

39

What action did you have to perform to convert isobutane to butane?

A. rotate around C2-C3 bondB. remove methyl group from C-2C. add one methyl group to C-1D. add one methyl group to C-2E. rotate around C1-C2

If you have to break bonds to interconvert isomers, they are constitutional

(structural) isomers


SlideLecture 3

Rotation Around Single Bonds

40

conformations: different spatial arrangements of atoms generated by rotation around single bonds

conformational analysis: comparison of the relative energies of different conformational isomers and how they in&uence properties and reactivity

lowest energy


SlideLecture 3

Measuring Relative Positions of Atoms

41

dihedral angle: angle between two intersecting planes;also called the torsion angle

plane can be de$ned by:

• 3 non-collinear points

• a line & a point not on that line

• two intersecting lines

• two parallel lines


SlideLecture 3

Eclipsed Conformation of Ethane

42

• C–H bonds on adjacent carbons are parallel (same plane)• H–C–C–H angle (dihedral angle) = 0º• highest energy conformation

H

H

0º


SlideLecture 3

Staggered Conformation of Ethane

43

• C–H bond bisects (cuts in half ) H–C–H angle on adjacent carbon• H–C–C–H angle (dihedral angle) = 60º• lowest energy conformation for ethane

HH

60º


SlideLecture 3

Drawing Conformations: Wedge & Dash

44

= group is pointing toward you, in front of the plane of paper

= group is pointing away from you, behind the plane of paper

= group is either toward or away from you, usually denotes mixtures

= group lies in the plane of the drawing surface

rarely followed convention:

thickest part of wedge or dash is always closest to

viewer

H

HHH

HH

H H

HH

HH


SlideLecture 3

Drawing Conformations: Sawhorse

45

• one C–C bond viewed “head-on” from oblique angle (acute or obtuse, but not 0º, 90º or 180º); skewed

• all atoms on central C–C bond are shown

H3CCH3

H3C HH CH3

H

H

H HH

H

H

CH3H

H H

H

HH3C

H3CCH3 H3C

CH3


SlideLecture 3

Why a Sawhorse?

46


SlideLecture 3

Drawing Conformations: Newman Projection

47

• one C–C bond viewed “head-on” at 0º angle• all atoms on central C–C bond are shown• circle represents back carbon atom

H3CCH3 H3C

CH3 H3CCH3

HH

H

H

HCH3

HH3C

H

H

CH3H

H

H

H

HHH3C


SlideLecture 3

Spatial Relationships in Staggered Conformations: Anti & Gauche

48

•anti: dihedral angle (torsion angle) = 180º•gauche: dihedral angle (torsion angle) = 60º•these relationships apply to any groups on adjacent carbon atoms

H

X

H

Y

H

HX

H

Y

HH

HH

X

Y

HHH

0 °60 °

180 °

Torsion Angle = 0°Eclipsed

Torsion Angle = 60°Gauche

Torsion Angle = 180°Anti-Periplannar


SlideLecture 3

Comparison of Conformational Drawings of Eclipsed and Staggered Ethane

49

Ball & Stick Newman Sawhorse Dash & Wedge

HH

H

H

HH

H

H

H

H

HH

H HH H

H

H

H

H

H H

H

H

H H

HH

HH

H

HHH

HH


SlideLecture 3

Self Test Question

50

What is the IUPAC name for molecule below?

A. 1,2,2,4,4-pentametnylhexaneB. 3,3,5,5-tetramethylheptaneC. 2-ethyl-2,4,4-trimethylhexaneD. 1,2,4,4-tetramethylhexaneE. 3,3-dimethyl-5,5-dimethylheptane

1

23

45

67

HH3C

H

CH3

H3C


SlideLecture 3

Self Test Question

51

Which set of molecules are conformational isomers?

A. aB. bC. cD. d

a.

b.

c.

d.

HH3C

H

CH3

CH3H3C

HH

CH3

CH3

HH3C

HCl

H

H

HH

ClH

H

H

HH

HH3C

CH3

H

H

HH3C

CH3

H

H

H

BrH

O

HHO

CH3 H

BrHO

H O

H

CH3

CHEM 494 Lecture 3ramsey1.chem.uic.edu/chem494/page7/files/CHEM 494 Lecture... · 2012. 9. 25. ·...

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Transcript of CHEM 494 Lecture 3ramsey1.chem.uic.edu/chem494/page7/files/CHEM 494 Lecture... · 2012. 9. 25. ·...