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Transcript of University of Illinois at Chicago UIC CHEM 494 Special Topics in Chemistry Prof. Duncan Wardrop...
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Prof. Duncan Wardrop
October 8, 2012
CHEM 494 - Lecture 5
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Chapter 15Alcohols and Alkyl
Halides
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Functional Groups Alcohols and Alkyl
Halides
UICUniversity of
Illinois at Chicago
CHEM 494, Spring 2010 SlideLecture 5: October 8
Functional Groupsfunctional 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
moleculesb.p. -88.6 °C
Inert to acids, bases, oxidizing & reducing agents
b.p. = 78.4 °C
Reacts with acids, bases, oxidizing & reducing agentsBiologically Active!
C CH
HH
H
HH C C
H
HO
H
HH
H
vs.
Ethane Ethanol
UICUniversity of
Illinois at Chicago
5CHEM 494, Spring 2010 SlideLecture 5: October 8
Examples of Functional Groups
alkenealkene alkynealkyne alcoholalcohol alkyl halidealkyl halide
C C C XC OH
OH
Cl
C C
H
UICUniversity of
Illinois at Chicago
6CHEM 494, Spring 2010 SlideLecture 5: October 8
Examples of Functional Groups
etherether sulfidesulfide thiolthiol amineamine
OCC C SH
SH
SCC C N
O
OMe
H2N NH2OS
UICUniversity of
Illinois at Chicago
7CHEM 494, Spring 2010 SlideLecture 5: October 8
Carbonyl Functional Groups: Carboxylic Acid Derivative
carbonyl grouppervasive common
name
carboxylic carboxylic acidacid esterester acid halideacid halide amideamide
acetylacet-
O
CO
H
O
CO
C
O
CX
O
CN
O
CO
HH3C
O
CO
CH2CH3H3C
O
CClH3C
O
CN
H
PhH3C
acetic acid ethyl acetate acetyl chloride acetanilide
C
O
C
O
CH3
UICUniversity of
Illinois at Chicago
8CHEM 494, Spring 2010 SlideLecture 5: October 8
Carbonyl Functional Groups
ketoneketonealdehydealdehyde
O
CH
O
CCH3H3CH3C
O
CH
O
CCC
acetaldehyde acetone
UICUniversity of
Illinois at Chicago
9CHEM 494, Spring 2010 SlideLecture 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
UICUniversity of
Illinois at Chicago
10
CHEM 494, Spring 2010 SlideLecture 5: October 8
IUPAC (Substitutive): Alkyl Halides
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.
3-bromo-5-methylheptane
parent parent locantlocant halo substituenthalo substituent parent chain nameparent chain name
CH3CH2CHCH2CHCH2CH3
Br CH3CH3Br
12
3 54 6
7
UICUniversity of
Illinois at Chicago
11
CHEM 494, Spring 2010 SlideLecture 5: October 8
IUPAC (Substitutive): Alcohols
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).
5-chloro-2,3-dimethyl-pentan-1-ol
locantlocant ““ol”ol”parent (drop last “e”)parent (drop last “e”)
5-chloro-2,3-dimethyl-1-pentanol
or
locantlocant ““ol”ol”parent (drop last “e”)parent (drop last “e”)
OH
Cl
12
3
4
5 CH2CH2CHCH3
Cl
CHCH2OHH3C
UICUniversity of
Illinois at Chicago
12
CHEM 494, Spring 2010 SlideLecture 5: October 8
Two Substitutive Nomenclatures vs. Functional Group Class Nomenclature
•substitutive and 2004 name are preferred
•old habits are hard to break
UICUniversity of
Illinois at Chicago
13
CHEM 494, Spring 2010 SlideLecture 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)
UICUniversity of
Illinois at Chicago
14
CHEM 494, Spring 2010 SlideLecture 5: October 8
Self Test Question
What is the IUPAC (substitutive) name for the following molecule?
A. 3-hydroxy-4,5,6-triethyl-7-bromooctane
B. 2-bromo-3,4,5-triethyloctan-3-ol
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
12
34
56
78
OH
Br
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Properties of Alcohols and Alkyl Halides
UICUniversity of
Illinois at Chicago
16
CHEM 494, Spring 2010 SlideLecture 5: October 8
Alcohols and Alkyl Halides Are Polar
red = higher electron density (partial negative
charge)
blue = lower/ deficient
electron density (partial positive
charge)
UICUniversity of
Illinois at Chicago
17
CHEM 494, Spring 2010 SlideLecture 5: October 8
Effect of Structure on Boiling Points
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: induced-dipole/induced-dipole
• induced/induced• dipole-dipole
attractive force• dipole/induced-
dipole
• induced/induced• dipole-dipole
attractive force• dipole/induced-
dipole• hydrogen bonding
UICUniversity of
Illinois at Chicago
18
CHEM 494, Spring 2010 SlideLecture 5: October 8
Review: induced-dipole//induced-dipole (London Dispersion Forces)
H
C
C
C
C
C
H H H H H H
H
HHHH
H
C
C
C
C
C
H H H H H H
H
HHHH
more atoms =
more electrons =
more induced dipoles =
more attractive forces =
higher boiling point
UICUniversity of
Illinois at Chicago
19
CHEM 494, Spring 2010 SlideLecture 5: October 8
Dipole/Induced-Dipole
•permanent dipole in one molecule induces a temporary dipole in a non-polar region of another molecule
•more C-X bonds = more dipole/dipole-induced attractive forces
C
C
F
H
H
H
H
H
C C
F
H
H
H
H
H
UICUniversity of
Illinois at Chicago
20
CHEM 494, Spring 2010 SlideLecture 5: October 8
Dipole/Dipole•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.
C
C
F
H
H
H
H
H
C
C
F
H
H H
H H
UICUniversity of
Illinois at Chicago
21
CHEM 494, Spring 2010 SlideLecture 5: October 8
Hydrogen Bonding is a Strong Dipole/Dipole Attractive Force
UICUniversity of
Illinois at Chicago
22
CHEM 494, Spring 2010 SlideLecture 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
UICUniversity of
Illinois at Chicago
23
CHEM 494, Spring 2010 SlideLecture 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
UICUniversity of
Illinois at Chicago
24
CHEM 494, Spring 2010 SlideLecture 5: October 8
Polarizability and Teflon
more bonds to F
(low polarizability) =
less/weaker induced-dipole/induced-dipole
attractive forces
Teflon® = polytetrafluoroethylene (PTFE)
n
F
F
F
F
F
F
F
F
F
F
F
F
UICUniversity of
Illinois at Chicago
25
CHEM 494, Spring 2010 SlideLecture 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
• polarizability increases down the periodic table; larger orbitals; more polarizable =
• better able to momentarily generate induced-dipole =
• stronger induced-dipole/induced-dipole forces =
• more attractive forces =
• higher boiling point
bromomethane(b.p. = 3 ºC)
fluoromethane(b.p. = -78 ºC)
UICUniversity of
Illinois at Chicago
26
CHEM 494, Spring 2010 SlideLecture 5: October 8
Self Test Question
Rank the following in order of increasing boiling point.
OH
F
Cl
F
F F
a.
b.
c.
d.
A. a, b, c, dB. b, c, d, aC. b, d, c, aD. a, c, b, dE. d, b, c, a
highest
lowest
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Chapter: 15
Organic ReactionsPreparation of Alkyl
Halides
UICUniversity of
Illinois at Chicago
28
CHEM 494, Spring 2010 SlideLecture 5: October 8
Preparation of Alkyl Halides from Alcohols
• 1°, 2° & 3° alcohols react
• irreversible reaction; no equilibrium here
• more reactive reactants & substrates = faster reaction
C OH H X+ + H O H
alcohol hydrogenhalide
alkyl halide water
C X
Substituition
UICUniversity of
Illinois at Chicago
29
CHEM 494, Spring 2010 SlideLecture 5: October 8
Reactivity of Hydrogen Halides
Increasing Reactivity of Hydrogen Halides Toward
Alcohols
HF HIHCl HBr<< <least reactive
(slowest)most reactive
(fastest)•stronger acid (lower pKa) = more reactive• increased reactivity = faster reaction• remember: irreversible reaction; no
equilibrium here
UICUniversity of
Illinois at Chicago
30
CHEM 494, Spring 2010 SlideLecture 5: October 8
Reactivity of Alcohols
•more substituted alcohol = more reactive• increased reactivity = faster reaction• remember: irreversible reaction; no
equilibrium here
UICUniversity of
Illinois at Chicago
31
CHEM 494, Spring 2010 SlideLecture 5: October 8
Higher Energy of Reactants = Increased Reactivity
higher energy reactants =
if transition states are the
same, then lower activation energy
(Ea) =
faster reaction = more reactive
CH3OH + HBr
CH3OH + HCl
CH3X + H2O
Ea Ea
= transition state
UICUniversity of
Illinois at Chicago
32
CHEM 494, Spring 2010 SlideLecture 5: October 8
Lower Energy Transition States = Increased Reactivity
higher energy transition states =
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.
+ HCl
RCl + H2O
Ea
Ea
= transition state
OH
OH
UICUniversity of
Illinois at Chicago
33
CHEM 494, Spring 2010 SlideLecture 5: October 8
More Examples
• think about the pattern of the reaction• ignore parts of the molecule that don’t react• reaction conditions: generally, above/below rxn
arrow
more reactive hydrogen halide (HBr) is needed for less reactive secondary alcohols
UICUniversity of
Illinois at Chicago
34
CHEM 494, Spring 2010 SlideLecture 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)
UICUniversity of
Illinois at Chicago
35
CHEM 494, Spring 2010 SlideLecture 5: October 8
1º & 2º Alcohols Unreactive Toward HCl
Alternative Reagent for Preparation of Alkyl Chlorides
thionyl chloride
SOCl2
• 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
OH H Cl+ Cl + H O Hno rxn
X
O
SClCl
OH Cl + SO2 HCl+SOCl2
K2CO3
UICUniversity of
Illinois at Chicago
36
CHEM 494, Spring 2010 SlideLecture 5: October 8
Alkyl Halides & Purple Pills
Esomeprazole
N
O
OH(SOCl2) N
O
Cl
O
SClCl
N
O
S
HN
N
N
HN
SH
MeO
N
O
S
HN
NO
S-Oxidation
S-Alkylation
Nexium®
UICUniversity of
Illinois at Chicago
37
CHEM 494, Spring 2010 SlideLecture 5: October 8
Self Test Question
Predict the organic product of the following reaction...
a. b. c.
d.
A. aB. bC. cD. dE. ee.
Cl
OH
H3C CH3
Br
Cl
H3C CH3
Cl
Cl
H3C CH3
Br
O
H3C CH3
Cl
Cl
Cl3C CCl3
Cl
Br
OH
H3C CH3
HCl
25 ºC ?
UICUniversity of
Illinois at Chicago
38
CHEM 494, Spring 2010 SlideLecture 5: October 8
Self Test Question
Which of the following transformations is unlikely to generate the product indicated?
A. a.
B. b.
C. c.
D.d.
a.
b.
c.
d.
xprimary
alcohols and HCl are
insufficiently reactive
HClOH Cl
OH
25 ºC
HCl
25 ºC Cl
OOH
SOCl2
K2CO3
OCl
Cl2
hν
Cl
UICUniversity of
Illinois at Chicago
39
CHEM 494, Spring 2010 SlideLecture 5: October 8
Substitution Reaction
hydroxyl group halide
Hydroxyl group is being substituted (replaced with) a
halide
R O H H X R X+ + H O H
alcohol hydrogenhalide
alkylhalide
water
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Chapter 15
Mechanisms of Substitution Reactions
UICUniversity of
Illinois at Chicago
41
CHEM 494, Spring 2010 SlideLecture 5: October 8
Substitution: How Does it Happen?
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
R O H H X R X+ + H O H
alcohol hydrogenhalide
alkyl halide water
break bond break bond make bond make bond
UICUniversity of
Illinois at Chicago
42
CHEM 494, Spring 2010 SlideLecture 5: October 8
Ingold-Hughes Mechanistic Designators
Letter#
N or E
Designatestype of
process
Designatesmolecularity
Nucleophilicor Electrophilic
Me
OHMe
MeH-Br
Me
BrMe
Me
Example
Rate = k x [t-BuOH]
SN1Substitution,nucleophilic,
1st order
UICUniversity of
Illinois at Chicago
43
CHEM 494, Spring 2010 SlideLecture 5: October 8
Nucleophilic Substitution (SN1)
Step OneProton Transfer (Protonation)
pKa = -3.9
OH
OH
HClH Cl
fast & reversible
alkyloxonium ion
UICUniversity of
Illinois at Chicago
44
CHEM 494, Spring 2010 SlideLecture 5: October 8
Step One Potential Energy Diagram
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
Step OneProton Transfer (Protonation)
UICUniversity of
Illinois at Chicago
45
CHEM 494, Spring 2010 SlideLecture 5: October 8
Mechanism: Nucleophilic Substitution (SN1)
Step TwoDissociation (Ionization)
OH
H MeMe
MeH2O
carbocation(t-butyl cation)
slow
UICUniversity of
Illinois at Chicago
46
CHEM 494, Spring 2010 SlideLecture 5: October 8
Step Two Potential Energy Diagram
•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.)
Step TwoDissociation (Ionization)
UICUniversity of
Illinois at Chicago
47
CHEM 494, Spring 2010 SlideLecture 5: October 8
Mechanism: Nucleophilic Substitution (SN1)
Step ThreeCarbocation Capture
t-butyl chloride
fastMeMe
Me
carbocation(t-butyl cation)
Cl Cl
Cation = Electrophile Anion = Nucleophile
UICUniversity of
Illinois at Chicago
48
CHEM 494, Spring 2010 SlideLecture 5: October 8
Step Three Potential Energy Diagram
•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)
Step ThreeCarbocation Capture
UICUniversity of
Illinois at Chicago
49
CHEM 494, Spring 2010 SlideLecture 5: October 8
Nucleophiles Add to Electrophilesnucleophile: 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!
MeMe
Me
Cl
Cation is Electrophileempty 2pz orbital
Chloride is Nucleophilefilled n orbital (: = lone pair)
UICUniversity of
Illinois at Chicago
50
CHEM 494, Spring 2010 SlideLecture 5: October 8
Complete Mechanism
OH
OH
HClH Cl
fast & reversible
alkyloxonium ion
MeMe
Me
H2Ocarbocation(t-butyl cation)
slow
t-butyl chloride
fastCl Cl
UICUniversity of
Illinois at Chicago
51
CHEM 494, Spring 2010 SlideLecture 5: October 8
Complete Potential Energy Diagram
•mechanism only valid for 3º & 2º alcohols
•reaction is only as fast as its slowest step
•slowest step (largest Ea) = rate determining step (RDS)
•here, slowest step is carbocation formation
•here, RDS is unimolecular
ProtonatioProtonationn
carbocation carbocation formationformation
carbocation carbocation capturecapture
rate determinin
g step (RDS)
UICUniversity of
Illinois at Chicago
52
CHEM 494, Spring 2010 SlideLecture 5: October 8
Naming the Mechanism: Ingold Notation
SN1S: Substitution N: Nucleophilic
1: 1st order (unimolecular)
R O H H X R X+ + H O H
alcohol hydrogenhalide
alkyl halide water
the alcohol functional groups
is being substituted with a
halide
the halide doing the substitution is
a nucleophile
the RDS is carbocation
formation; this step is
unimolecular (1st order)
UICUniversity of
Illinois at Chicago
53
CHEM 494, Spring 2010 SlideLecture 5: October 8
Self Test Question
Consider the SN1 mechanism for the formation of 2-bromobutane. Which structure best represents the highest energy transition state in this mechanism?
A. a.
B. b.
C. c.
D.d.
a.
b.
c.
d.
OH
H Br
OH H
+ Br
OH H
+Br
OH H
+ Br
δ+ δ−
δ+
δ+
H