Transcript CHEM 494 Lecture 5 - UIC Department of Chemistry

CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
CHEM 494 - Lecture 5
Prof. Duncan Wardrop
October 8, 2012
CHEM 494
University of
Special Topics in Chemistry Illinois at Chicago
UIC
Chapter 15
Alcohols and Alkyl
Halides
CHEM 494
University of
Special Topics in Chemistry Illinois at Chicago
UIC
Functional Groups
Alcohols and Alkyl
Halides
Functional Groups
functional group: a defined connectivity for a
specific group of atoms (≥2) within a molecule
since alkanes are chemically inert, functional groups are
responsible for chemical reactivity under specific
conditions and also the physical, chemical and biological
properties of organic molecules
b.p. -88.6 °C
H
H C C H
Inert to acids,
H
H
bases, oxidizing &
reducing agents
University of
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H
H
Ethane
vs.
H
H C C O
H
H H
Ethanol
CHEM 494, Spring 2010
b.p. = 78.4 °C
Reacts with acids,
bases, oxidizing &
reducing agents
Biologically Active!
Slide
Lecture 5: October 8
Examples of Functional Groups
alkene
alkyne
C C
C C
alcohol
alkyl halide
C X
C OH
Cl
H
OH
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CHEM 494, Spring 2010
Slide 5
Lecture 5: October 8
Examples of Functional Groups
ether
C
O
sulfide
C
C
S
C
thiol
amine
C N
C SH
OMe
S
O
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O
CHEM 494, Spring 2010
SH
H2N
NH2
Slide 6
Lecture 5: October 8
Carbonyl Functional Groups:
Carboxylic Acid Derivative
O
pervasive
common
name
O
C
C
carbonyl group
carboxylic acid
ester
O
O
C
O
H
C
H3C
C
acid halide
O
H
H3C
C
amide
O
O
C
O
C
O
O
acetyl
CH3 acet-
C
X
O
O
CH2CH3
H3C
C
N
O
Cl
H3C
C
N
Ph
H
acetic acid
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ethyl acetate
acetyl chloride
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acetanilide
Slide 7
Lecture 5: October 8
Carbonyl Functional Groups
aldehyde
H3 C
O
O
C
C
H
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C
O
O
C
C
H
acetaldehyde
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ketone
H3 C
C
CH3
acetone
CHEM 494, Spring 2010
Slide 8
Lecture 5: October 8
Time to Memorize Functional Groups!
• many already encountered alkenes, alkynes, arenes not
alkanes
• study functional group
handout from website - learn
to draw & name F.G.s
• make flash cards
• you will be asked to identify
and name functional groups
on quiz 3 & first exam
University of
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CHEM 494, Spring 2010
Slide 9
Lecture 5: October 8
IUPAC (Substitutive): Alkyl Halides
Br
1
2
CH3
4
3
5
6
7
Br
CH3
CH3CH2CHCH2CHCH2CH3
3-bromo-5-methylheptane
Steps:
1.Identify and number the longest continuous chain of carbons.
2.Follow all previous rules and conventions for naming/numbering alkane chains.
3.Name the compound according to the figure below.
Conventions:
•Previous conventions apply (e.g., first point of difference rule).
•Halogens and alkyl groups are considered to have equal rank when deciding numbering. If two
numbering schemes give same locant, choose numbering that lists substituents alphabetically.
•Subsituent names for halogens are fluoro, chloro, bromo, & iodo.
parent
locant
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halo substituent
parent chain name
CHEM 494, Spring 2010
Slide 10
Lecture 5: October 8
IUPAC (Substitutive): Alcohols
4
Cl
Cl
CH2CH2CHCH3
3
5
2
OH
H3C CHCH2OH
1
5-chloro-2,3-dimethyl-pentan-1-ol
or
5-chloro-2,3-dimethyl-1-pentanol
Steps:
1.Identify and number the longest continuous chain of carbons to give the -OH group the lowest locant.
2.Name the parent by replacing -e with -ol (e.g. pentane becomes pentanol).
3.Name the compound according to the figure below.
Conventions:
•Previous conventions apply (e.g., first point of difference rule).
•Alcohols outrank (have priority over) halogens and alkyl groups when considering numbering scheme.
•Alcohol locant may be placed before the parent name (e.g. 1-pentanol) or after ( e.g. pent-1-ol).
locant
parent (drop last “e”)
parent (drop last “e”)
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locant
CHEM 494, Spring 2010
“ol”
“ol”
Slide 11
Lecture 5: October 8
Two Substitutive Nomenclatures vs.
Functional Group Class Nomenclature
• substitutive and 2004 name are preferred
• old habits are hard to break
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CHEM 494, Spring 2010
Slide 12
Lecture 5: October 8
Classification of Substituted Carbons
• count the number of carbons bonded to
the carbon atom you wish to classify:
• one = primary (1º)
• two = secondary (2º)
• three = tertiary (3º)
• four = quaternary (4º)
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CHEM 494, Spring 2010
Slide 13
Lecture 5: October 8
Self Test Question
What is the IUPAC (substitutive)
name for the following molecule?
8
5
1
7 Br
A. 3-hydroxy-4,5,6-triethyl-7bromooctane
6
B. 2-bromo-3,4,5-triethyloctan-3-ol
3
4
2
OH
C. 7-bromo-4,5,6-triethyloctan-3-ol
D. 4-(3-bromo-1,2-diethylbutyl)-3-hexanol
E. 6-(1-bromoethyl)-4,5-diethyl-3-octanol
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CHEM 494, Spring 2010
Slide 14
Lecture 5: October 8
CHEM 494
University of
Special Topics in Chemistry Illinois at Chicago
UIC
Properties of Alcohols
and Alkyl Halides
Alcohols and Alkyl Halides Are Polar
red = higher
electron density
(partial negative
charge)
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blue = lower/
deficient electron
density (partial
positive charge)
CHEM 494, Spring 2010
Slide 16
Lecture 5: October 8
Effect of Structure on Boiling Points
• induced/induced
• dipole-dipole attractive
force
• dipole/induced-dipole
• induced/induced
• dipole-dipole attractive
force
• dipole/induced-dipole
• hydrogen bonding
CH3CH2CH3
(propane)
CH3CH2F
(fluoroethane)
CH3CH2OH
(ethanol)
MW
44
48
46
boiling point
(ºC)
-42
-32
+78
Dipole
moment (µ)
0
1.9
1.7
• only London
dispersion forces
• aka: induceddipole/induced-dipole
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CHEM 494, Spring 2010
Slide 17
Lecture 5: October 8
Review: induced-dipole//induced-dipole
(London Dispersion Forces)
H
H
H
C
H
H
C
H
more atoms =
C
C
H
H
C
H
H
H
more electrons =
H
more induced dipoles =
H
H
H
C
H
C
H
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C
H
H
more attractive forces =
C
C
H
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H
H
H
higher boiling point
H
CHEM 494, Spring 2010
Slide 18
Lecture 5: October 8
Dipole/Induced-Dipole
H
H
H
C
C
H
H
F
H
H
H
C
C
H
H
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F
• permanent dipole in
one molecule induces
a temporary dipole in a
non-polar region of
another molecule
• more C-X bonds =
more dipole/dipoleinduced attractive
forces
CHEM 494, Spring 2010
Slide 19
Lecture 5: October 8
Dipole/Dipole
H
H
H
C
C
H
H
F
H
H
F
C
C
H
H
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H
• attractive force
between two
permanent dipoles
• not necessarily only at
the polar covalent
bond: region of partial
positive charge in one
molecule attracted to
region of negative
charge in another
molecule.
CHEM 494, Spring 2010
Slide 20
Lecture 5: October 8
Hydrogen Bonding is a Strong
Dipole/Dipole Attractive Force
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Slide 21
Lecture 5: October 8
Boiling Point Trends
• in same class (i.e. X = F) boiling point increases as MW increases; more atoms
= more attractive van der Waals forces = higher boiling point
• alcohols have significantly higher boiling points than similar halides; strong
hydrogen bonding attractive forces
• boiling point increases from fluorine to iodine for same series; polarizability of
halogen increases down periodic table
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Slide 22
Lecture 5: October 8
Boiling Point Trends (Cl, Br, I Only)
boiling point increases with increasing number of halogens
• despite CCl4 having no molecular dipole, it has the highest boiling point
• induced-dipole/induced-dipole forces are the greatest because it has the
greatest number of chlorine atoms
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CHEM 494, Spring 2010
Slide 23
Lecture 5: October 8
Polarizability and Teflon
Teflon® = polytetrafluoroethylene (PTFE
F F F F F F
F F F F F F
n
more bonds to F
(low polarizability) =
less/weaker induceddipole/induced-dipole
attractive forces
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CHEM 494, Spring 2010
Slide 24
Lecture 5: October 8
Polarizability
polarizability: the ease of distortion of the electron cloud of a
molecular entity by an electric field; “flexibility”, “squishiness”
of the electron cloud
bromomethane
(b.p. = 3 ºC)
fluoromethane
(b.p. = -78 ºC)
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•
polarizability increases down
the periodic table; larger
orbitals; more polarizable =
•
better able to momentarily
generate induced-dipole =
•
stronger induceddipole/induced-dipole forces =
• more attractive forces =
• higher boiling point
CHEM 494, Spring 2010
Slide 25
Lecture 5: October 8
Self Test Question
Rank the following in order of
increasing boiling point.
a.
OH
b.
c.
F
d.
F
Cl
F F
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highest
lowest
A. a, b, c, d
B. b, c, d, a
C. b, d, c, a
D. a, c, b, d
E. d, b, c, a
CHEM 494, Spring 2010
Slide 26
Lecture 5: October 8
CHEM 494
University of
Special Topics in Chemistry Illinois at Chicago
UIC
Organic Reactions
Preparation of Alkyl
Halides
Chapter: 15
Preparation of Alkyl Halides from Alcohols
Substituition
C OH
+
alcohol
•
•
•
H X
C X
hydrogen
halide
+
alkyl halide
H O H
water
1°, 2° & 3° alcohols react
irreversible reaction; no equilibrium here
more reactive reactants & substrates = faster
reaction
University of
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CHEM 494, Spring 2010
Slide 28
Lecture 5: October 8
Reactivity of Hydrogen Halides
Increasing Reactivity of
Hydrogen Halides Toward
Alcohols
HF
<
HCl
<
HBr
<
HI
least reactive
most reactive
(slowest)
(fastest)
• stronger acid (lower pKa) = more reactive
• increased reactivity = faster reaction
• remember: irreversible reaction; no equilibrium
here
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CHEM 494, Spring 2010
Slide 29
Lecture 5: October 8
Reactivity of Alcohols
• more substituted alcohol = more reactive
• increased reactivity = faster reaction
• remember: irreversible reaction; no equilibrium
here
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CHEM 494, Spring 2010
Slide 30
Lecture 5: October 8
Higher Energy of Reactants =
Increased Reactivity
CH3OH + HBr
Ea
Ea
higher energy
reactants =
if transition states
are the same, then
lower activation
energy (Ea) =
CH3OH + HCl
= transition state
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CH3X + H2O
CHEM 494, Spring 2010
faster reaction
= more reactive
Slide 31
Lecture 5: October 8
Lower Energy Transition States =
Increased Reactivity
higher energy transition
states =
Ea
OH
Ea
+ HCl
OH
= transition state
RCl + H2O
if reactant energies are
close, then lower
activation energy (Ea) =
faster reaction
= more reactive
We will explore why 3º alcohols provide lower energy transition states on Thursday.
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Slide 32
Lecture 5: October 8
More Examples
more reactive hydrogen halide (HBr) is needed for less reactive secondary
alcohols
•
•
•
think about the pattern of the reaction
ignore parts of the molecule that don’t react
reaction conditions: generally, above/below rxn arrow
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CHEM 494, Spring 2010
Slide 33
Lecture 5: October 8
Alternative Conditions
A mixture of sodium bromide and
sulfuric acid may be used in place of
HBr.
•
•
reagents generally placed above/below reaction
arrow
inorganic products usually omitted (assumed)
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CHEM 494, Spring 2010
Slide 34
Lecture 5: October 8
1º & 2º Alcohols Unreactive Toward HCl
no rxn
OH
+
X
H Cl
Cl
+
H O H
Alternative Reagent for Preparation of Alkyl Chlorides
thionyl chloride
Cl
Cl
SOCl2
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Cl + SO2 + HCl
OH
K2CO3
O
S
SOCl2
UIC
• thionyl chloride reacts rapidly with 1º and 2º alcohols
• byproducts of the reaction are SO2(g) and HCl(g)
• base is needed to neutralize HCl: e.g. K2CO3, pyridine
CHEM 494, Spring 2010
Slide 35
Lecture 5: October 8
Alkyl Halides & Purple Pills
H
N
O
O
Cl
OH
N
S
O
SH
Cl
(SOCl2)
O
N
MeO
Cl
N
S-Alkylation
O
S-Oxidation
N
N
S
N
HN
O
S
N
HN
Esomeprazole
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CHEM 494, Spring 2010
Nexium®
Slide 36
Lecture 5: October 8
Self Test Question
Predict the organic product of the following
reaction...
Br
HCl
?
OH
25 ºC
H3C CH3
a.
OH
b.
Cl
O
H3C CH3
UIC
Cl
H3C CH3
Cl
Br
d.
c.
H3C CH3
H3C CH3
University of
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Cl
Br
Cl
Cl
e.
Cl
A. a
B. b
C. c
D. d
E. e
Cl3C CCl3
CHEM 494, Spring 2010
Slide 37
Lecture 5: October 8
Self Test Question
Which of the following transformations is unlikely
to generate the product indicated?
OH
a.
primary alcohols
and HCl are
insufficiently
reactive
x
25 ºC
b.
d.
UIC
Cl
A. a.
HCl
c.
University of
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HCl
OH
O
SOCl2
OH
Cl
25 ºC
O
K2CO3
Cl2
Cl
Cl
B. b.
C. c.
D. d.
h
CHEM 494, Spring 2010
Slide 38
Lecture 5: October 8
Substitution Reaction
hydroxyl group
R O H
halide
+
alcohol
H X
R X
hydrogen
halide
alkyl
halide
+
H O H
water
Hydroxyl group is being substituted
(replaced with) a halide
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CHEM 494, Spring 2010
Slide 39
Lecture 5: October 8
CHEM 494
University of
Special Topics in Chemistry Illinois at Chicago
UIC
Mechanisms of
Substitution Reactions
Chapter 15
Substitution: How Does it Happen?
break bond
R O H
+
alcohol
break bond
make bond
H X
R X
hydrogen
halide
alkyl halide
make bond
+
H O H
water
mechanism: a generally accepted series of elementary steps that
show the order of bond breaking and bond making
elementary step: a bond making and/or bond breaking step that only
involves one transition state
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CHEM 494, Spring 2010
Slide 41
Lecture 5: October 8
Ingold-Hughes Mechanistic Designators
Designates
molecularity
#
Designates
type of
process
Letter
N or E
Nucleophilic
or
Electrophilic
Example
Me
Me
Me
Me
H-Br
OH
Me
Me
Br
S N1
Substitution,
nucleophilic,
1st order
Rate = k x [tBuOH]
University of
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CHEM 494, Spring 2010
Slide 42
Lecture 5: October 8
Nucleophilic Substitution (SN1)
Step One
Proton Transfer (Protonation)
fast & reversible
H
O
H Cl
O
H
H
pKa = -3.9
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Cl
alkyloxonium
ion
CHEM 494, Spring 2010
Slide 43
Lecture 5: October 8
Step One Potential Energy Diagram
Step One
Proton Transfer (Protonation)
transition state: energy
maximum along reaction
coordinate for one elementary
step; usually involves partial
bond making and partial bond
breaking
intermediate: energy
minimum along the reaction
coordinate; species with a
finite lifetime; neither reactant,
nor product
Hammond Postulate:
structure of the transition state
“looks” most like its closest
energy reactant or
intermediate
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CHEM 494, Spring 2010
Slide 44
Lecture 5: October 8
Mechanism: Nucleophilic Substitution (SN1)
Step Two
Dissociation (Ionization)
H
slow
Me
Me
O
H2O
H
Me
carbocation
(t-butyl cation)
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CHEM 494, Spring 2010
Slide 45
Lecture 5: October 8
Step Two Potential Energy Diagram
Step Two
Dissociation (Ionization)
• largest activation energy (Ea)
• endothermic, slowest
• carbocation intermediate is
much higher in energy than
an oxonium ion
• carbocations do not have a
full octet, whereas oxonium
ions do
• structure of transition state
most resembles the closest
energy neighbor, the
carbocation (Hammond Post.)
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CHEM 494, Spring 2010
Slide 46
Lecture 5: October 8
Mechanism: Nucleophilic Substitution (SN1)
Step Three
Carbocation Capture
fast
Me
Me
Cl
Cl
Me
carbocation
(t-butyl cation)
t-butyl chloride
Cation = Electrophile
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Anion = Nucleophile
CHEM 494, Spring 2010
Slide 47
Lecture 5: October 8
Step Three Potential Energy Diagram
Step Three
Carbocation Capture
• fast step because small
activation energy; positive and
negative atoms bond fast
• products are much lower in
energy since they are neutral;
exothermic reaction
• transition state looks most like
its closest neighbor, the
carbocation intermediate (very
little C-Cl bond formation at
transition state) (Hammond
Postulate)
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CHEM 494, Spring 2010
Slide 48
Lecture 5: October 8
Nucleophiles Add to Electrophiles
nucleophile: nucleus loving; Lewis base; electron pair donor; forms
bonds with a nucleus that can accept electrons; does not
necessarily have to be negatively charged; has available, filled
orbitals!
electrophile: electron loving; Lewis acid; electron pair acceptor;
forms bonds by accepting electrons from other atoms; does not
necessarily have to be positively charged; has available, empty
orbitals!
Me
Cation is Electrophile
empty 2pz orbital
Me
Cl
Me
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Chloride is Nucleophile
filled n orbital (: = lone pair)
CHEM 494, Spring 2010
Slide 49
Lecture 5: October 8
Complete Mechanism
fast & reversible
H
O
H Cl
O
H
Cl
H
alkyloxonium
ion
slow
Me
Me
fast
Cl
Cl
Me
carbocation
(t-butyl cation)
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H2O
CHEM 494, Spring 2010
t-butyl chloride
Slide 50
Lecture 5: October 8
Complete Potential Energy Diagram
• mechanism only valid for 3º
& 2º alcohols
• reaction is only as fast as
its slowest step
rate
determining
step (RDS)
• slowest step (largest Ea) =
rate determining step
(RDS)
• here, slowest step is
carbocation formation
Protonation carbocation formation carbocation capture
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CHEM 494, Spring 2010
• here, RDS is unimolecular
Slide 51
Lecture 5: October 8
Naming the Mechanism: Ingold Notation
R O H
+
alcohol
+
H X
R X
hydrogen
halide
alkyl halide
H O H
water
SN1
S: Substitution
the alcohol
functional groups is
being substituted
with a halide
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N: Nucleophilic
the halide doing the
substitution is a
nucleophile
CHEM 494, Spring 2010
1: 1st order
(unimolecular)
the RDS is
carbocation
formation; this step
is unimolecular (1st
order)
Slide 52
Lecture 5: October 8
Self Test Question
Consider the SN1 mechanism for the formation of 2bromobutane. Which structure best represents the
highest energy transition state in this mechanism?
a.
O
H 
H
Br

Br
+
c.
H
H
O
University of
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O
H H

UIC
H
B. b.
C. c.

b.
A. a.
+
Br
d.
H
O
CHEM 494, Spring 2010
+
Br
D. d.
H
Slide 53
Lecture 5: October 8