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



Chapter 15Alcohols and Alkyl

Halides



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

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Examples of Functional Groups

etherether sulfidesulfide thiolthiol amineamine

OCC C SH

SH

SCC C N

O

OMe

H2N NH2OS

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Illinois at Chicago


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

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Illinois at Chicago


Carbonyl Functional Groups

ketoneketonealdehydealdehyde

O

CH

O

CCH3H3CH3C

O

CH

O

CCC

acetaldehyde acetone

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Illinois at Chicago


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

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10


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

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Illinois at Chicago

11


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

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Illinois at Chicago

12


Two Substitutive Nomenclatures vs. Functional Group Class Nomenclature

•substitutive and 2004 name are preferred

•old habits are hard to break

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13


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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14


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



Properties of Alcohols and Alkyl Halides

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16


Alcohols and Alkyl Halides Are Polar

red = higher electron density (partial negative

charge)

blue = lower/ deficient

electron density (partial positive

charge)

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17


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

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18


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

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19


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

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Illinois at Chicago

20


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

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21


Hydrogen Bonding is a Strong Dipole/Dipole Attractive Force

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22


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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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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24


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

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Illinois at Chicago

25


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)

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26


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



Chapter: 15

Organic ReactionsPreparation of Alkyl

Halides

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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

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29


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

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Reactivity of Alcohols

•more substituted alcohol = more reactive• increased reactivity = faster reaction• remember: irreversible reaction; no

equilibrium here

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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

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32


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

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Illinois at Chicago

33


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

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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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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

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36


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®

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Illinois at Chicago

37


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 ?

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Illinois at Chicago

38


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

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Illinois at Chicago

39


Substitution Reaction

hydroxyl group halide

Hydroxyl group is being substituted (replaced with) a

halide

R O H H X R X+ + H O H


alkylhalide

water



Chapter 15

Mechanisms of Substitution Reactions

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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



alkyl halide water

break bond break bond make bond make bond

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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

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Nucleophilic Substitution (SN1)

Step OneProton Transfer (Protonation)

pKa = -3.9

OH

OH

HClH Cl

fast & reversible

alkyloxonium ion

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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)

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Mechanism: Nucleophilic Substitution (SN1)

Step TwoDissociation (Ionization)

OH

H MeMe

MeH2O

carbocation(t-butyl cation)

slow

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46


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)

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Mechanism: Nucleophilic Substitution (SN1)

Step ThreeCarbocation Capture

t-butyl chloride

fastMeMe

Me

carbocation(t-butyl cation)

Cl Cl

Cation = Electrophile Anion = Nucleophile

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48


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

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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)

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50


Complete Mechanism

OH

OH

HClH Cl

fast & reversible

alkyloxonium ion

MeMe

Me

H2Ocarbocation(t-butyl cation)

slow

t-butyl chloride

fastCl Cl

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51


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)

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52


Naming the Mechanism: Ingold Notation

SN1S: Substitution N: Nucleophilic

1: 1st order (unimolecular)



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)

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53


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

university of illinois at chicago uic chem 494 special topics in chemistry prof. duncan wardrop...

Documents

slide lecture

chicago chem

chicago uic chem

uic university of illinois

exam slide

alkyl groups

previous conventions

alkyl halides steps