CHEM 494 Lecture 2ramsey1.chem.uic.edu/chem494/page7/files/CHEM 494 Lecture 2.pdfUniversity of UIC...

University of Illinois at ChicagoUICCHEM 494

Special Topics in Chemistry

Prof. Duncan WardropSeptember 17, 2012

CHEM 494 - Lecture 2

UICUniversity of Illinois at Chicago CHEM 494, Fall 2012

SlideLecture 2

Course Website

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

SyllabusCourse PoliciesOther handoutsAnnouncements (Course News)Course Calendar


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Periodicity

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Periodicity

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Lecture Summary

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Revision & Proof of Resonance

Introduction to Hydrocarbons

Classi!cation

Structure & Bonding


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Revision of Resonance

6

Do not confuse equilibrium with resonance: double-headed arrows represent resonance contributors; two separate arrows represent the equilibrium of distinct chemical species....

O

MeO

O

MeO

Equilibrium(atoms move)

O

H

OH

Resonance(electrons move)

HH


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Prioritizing Resonance Contributors(How much Unicorn?!)

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Priority 1No !rst-row elements (B, C, N, O) can have more than eight electrons in its valence shell.....

NO

ONO

ONO

O

EqualContributor

EqualContributor

Non-PermissableContributor

(N has 10 elect.)


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Prioritizing Resonance Contributors

8

Priority 2Resonance structures in which all atoms are surrounded by an octet of electrons are almost always lower in energy than those resonance structures in which one or more atoms are electron de!cient.

OH

OH8 electrons

8 electrons

6 electrons

MoreStable

OH

120 pm (C=O)128 pm (calculated)143 pm (C-O)

C-O Bond Length


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Priority 3Resonance structures with charge separation are typically higher in energy than those without. If charge separation exists, then electropositive atoms (C, B) are better able to bear + charge...

H HH H

H H

UnimportantContributor

MajorContributor

PhysicalEvidence

0.66 D

Small DipoleMoment


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Priority 4If charge separation exists in a resonance structure, then the electronegative atoms should gain the formal negative charge and vice versa:

4.02 D

O O

MajorContributor

Minor(but significant)

Contributor

O

UnimportantContributor


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Self Test Question

11

Rank the following acids in order of increasing acidity.

A. a, c, b, e, dB. e, c, a, b, dC. c, e, a, d, bD. b, a, c, d, eE. d, b, a, c, e

OH

O

OH

OH

O

O+ H

O + H

O + H

SH

O

S

O+ H

HN N + H

a.

b.

c.

d.

e.


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Self Text Question

12

Which is the most acidic proton in the molecule below?

A. aB. bC. cD. dE. e

O

OH

H HO

H

HH

HH

a

b

c

d

O He



Classes, Bonding & Structural Features of

Hydrocarbons


SlideLecture 2

Hydrocarbons

Aliphatic

Alkanes

Classes of Hydrocarbons

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• aleiphar Greek for “fat”• sources were fats and oils

• only single bonds• nomenclature: -ane suffix

Alkenes Alkynes

• contain a double bond• nomenclature: -ene suffix

• contain a triple bond• nomenclature: -yne suffix

Aromatic

• only C & H atoms• framework for funct. groups• generally, non-reactive

• aka: arenes• many isolated from plants;

impurities can be fragrant• aromaticity de"nition: Ch. 12


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Alkanes

Examples of Aliphatic Hydrocarbons

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• only single bonds• CnH2n+2

• nomenclature: -ane suffix

Alkenes Alkynes• contain a double bond•CnH2n (1 double bond)•nomenclature: -ene

suffix

• contain a triple bond•CnH2n-2 (1 triple bond)•nomenclature: -yne

suffix

C CH

HC

H

HC H

H

HH

H

H

CH3CH2CH2CH3

C CH

CH

HCHH

H H

H

CH3CH2CHCH2

C C CH

HCHH

HH

CH3CH2CCH

butane 1-butene 1-butyne


SlideLecture 2

Examples of Aromatic Hydrocarbons

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Aromatic• aka: arenes• many isolated from plants;

impurities were fragrant• aromaticity de"nition: Ch.

12

CCCCC

C

CH3 N O

HH

HH

H

H

benzene benzene

toluene napthalene pyridine furan

• many are derivatives of benzene

• resonance: delocalized electrons

• adjacent double bonds• planar (120º bond angles)• polyaromatic hydrocarbons

(PAHs) contain two or more aromatic rings; highly carcinogenic and environmental toxin


SlideLecture 2

Structural Features of Simple Alkanes

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• 4 bonds to carbon, 1 bond to H• C–H bond length ~ 110 pm• C–C bond length ~ 153 pm• all bond angles are nearly tetrahedral


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Initial Valence Bond Model of Carbon

18

2s2p 2p 2p

1s 1s

C

H H

CH H

• carbon only has two half-!lled orbitals• should, then, form two bonds• actually forms 4 bonds!?!?


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Valence Bond Model: Hybridization

19

2s

2p 2p 2p

sp3 sp3 sp3 sp3

mix s & p orbitals

• hybrids: mixture of s & p; sp3 lower in energy than p• 25% s-character and 75% p-character• 4 degenerate orbitals = equal in energy• each half-"lled, can form 4 bonds• VSEPR: tetrahedral (bond angles = 109.5º)


SlideLecture 2

+ –+

Understanding the Shape of sp3 Orbitals

20

+

+ –

+ –

destructiveinterference

constructiveinterference

• constructive interference: reinforce electron wave where sign is the same• destructive interference: cancel out wave where sign is opposite• higher probability of "nding an electron on one side of the nucleus

x3


SlideLecture 2

Sigma Bonds (σ-bond) in Methane

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• σ-bond: head-to-head overlap of orbitals; strongest bond type• hybridization increases bond strength of σ-bonds since electrons are

concentrated on one side of nucleus• only larger lobe of sp3 involved in bonding• typically ignore (don’t draw) smaller lobe

109.5º

C H

H

HH

= 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

C(2sp3)––H(1s)σ-bond


SlideLecture 2

The C–C σ-bond in Ethane

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• in-phase overlap of an sp3-hybridized orbital from each carbon atom• overlap is along internuclear axis (σ-bond)• VSEPR: tetrahedral around each carbon atom



Nomenclature of Hydrocarbons


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What’s In a Name?

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N

O

CH3

CH3H3C

IUPAC: ethyl (3R,4R,5S)-5-amino-4-acetamido-3-(pentan-3-yloxy)cyclohex-1-ene-1-carboxylateGeneric: oseltamivirTrade: Tamiflu

IUPAC: N,N-diethyltoluamideInitialism: DEETTrade: OFF

IUPAC: sodium dodecyl sulfateCommon: sodium lauryl sulfateTrade: SDS

IUPAC: Sodium (2S)-2-amino-5-hydroxy-5-oxo-pentanoateCommon: mono sodium glutamateInitialism: MSG

O

O

OClHO

HOCl

HO

ClOH

OH

IUPAC: 1,6-Dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranosideCommon: sucraloseTrade: Splenda


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IUPAC: Alkanes with NO Substituents

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IUPAC: International Union of Pure and Applied Chemistry

common name

retained

Greekprefixes


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Ide

nti

cal

Constitutional (Structural) Isomers

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C9H20

C9H20 C9H20

C9H20

constitutional isomers: same molecular formula; different connectivity

connectivity: order in which the atoms are bonded


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Common Names for Branched Isomers of Simple Alkanes

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n-butane n-pentane n-hexane

iso-butane iso-pentane iso-hexane

neo-pentane neo-hexane

“normal”straight chain

isoCH3

CHH3C

n neoCH3

CH3C CH3

homologous series: successive members differ by a –CH2– group

methyl: CH3

methylene: CH2

methine: CH


SlideLecture 2

Common Names Become Too Complex:Systematic Method of Naming is Needed

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IUPAC: Naming Alkyl Substituent Groups

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H3CCCCC

H H

H3C H

H HCH3

H H

substituent: atom or group other than H bonded to carbonalkyl group: one H atom less than an alkane

Steps:1. Identify longest continuous chain of carbon atoms.2. Locate substituents attached to the longest continuous chain of carbon atoms.3. Name alkane substituent by replacing -ane of parent alkane with -yl

CH3CH2CHCH2CH2CH3CH3

CH3CH3Ethane

CH3(CH2)5CH3Heptane

CH3(CH2)16CH3Octadecane


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Common & IUPAC Names of 3-Carbon Alkyl Groups

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Common & IUPAC Names of 4-Carbon Alkyl Groups

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Carbon Classification

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Physical Properties of Alkanes


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Crude Oil - Source of Alkanes

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cracking: (chemical change)converts high molecular weight hydrocarbons to more useful, lower molecular weight fragments

reforming: (chemical change)increases branching of the hydrocarbon chains; branched hydrocarbons have better burning characteristics

distillation: (physical separation)increases branching of the hydrocarbon chains; branched hydrocarbons have better burning characteristics


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Oil Refining

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Boiling Points Increase with # of Carbons

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• boiling point increases with increasing number of carbon atoms

• branched alkanes have lower boiling points than unbranched isomers


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Intermolecular Attractive Forces

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van der Waals forces (VWF): intermolecular attraction between neutral species1. dipole-dipole (including hydrogen bonding)2. dipole/induced-dipole3. induced-dipole/induced-dipole (London dispersion)

• alkanes are non-polar; dipole-dipole & dipole/induced-dipole forces absent

X

X

✓

• only forces of attraction between non-polar compounds are induced-dipole/induced-dipole


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Induced-Dipole/Induced-Dipole(London Dispersion Forces)

38

• green = electric !eld of surrounding two covalently bonded nuclei

• a.k.a. cumulative electron cloud

initial picture: centers of positive and negative charge in each individual molecule are identical


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induced dipole in left molecule: movement of electrons creates instantaneous dipole in left molecule

• constant motion of electrons in orbitals• blue/red: separation of positive and negative areas of

charge within the electric !eld of molecule• caused by asymmetric distribution of electrons


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left dipole induces right dipole: electron cloud in right molecule spontaneously adjusts to complement left

• electric attraction between two molecules• both electric "elds $uctuate, but always in the direction

that produces weak attraction• more atoms = more electrons = more induced dipoles =

more attractive forces = higher boiling points

attractive force


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HC

CC

CC

H H H H H H

H

HHHH

HC

CC

CC

H H H H H H

H

HHHH

more atoms =

more electrons =

more induced dipoles =

more attractive forces =

higher boiling point


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London VWFs in Branched Alkanes

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Increasing London Dispersion Forces

Increasing Surface Area

unbranched highly branched

Gas at RTLiquid at RT


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Contribution of London Forces toIntermolecular Interaction Energy

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Molecule Pair % of the Total Interaction Energy

H2O-H2O 24

NH3-NH3 57

HCl-HCl 86

HBr-HBr 96

HI-HI 99

Ne-Ne 100

CH4-CH4 100

CHEM 494 Lecture 2ramsey1.chem.uic.edu/chem494/page7/files/CHEM 494 Lecture 2.pdfUniversity of UIC...

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