Chemistry 121(01) Winter 2010-11 Introduction to Organic Chemistry and Biochemistry Instructor Dr. Upali Siriwardane (Ph.D. Ohio State) E-mail: [email protected] Office: 311 Carson Taylor Hall ; Phone: 318-257-4941; Office Hours: MWF 8:00 am - 10:00 am; TT 9:00 – 10:00 am & 1:00-2:00 pm. December 17, 2010 Test 1 (Chapters 12-13) January 19, 2011 Test 2 (Chapters 14,15 & 16) February 7, 2011 Test 3(Chapters 17, 18 & 19) February 23, 2011 Test 4 (Chapters 20, 21 & 22) February 24, 2011 Comprehensive Make Up Exam:

Chemistry 121, Winter 2009-10, LA Tech

Sections 12.4-12.14 & 12.6

12-1

Alkanes: Acyclic Saturated Hydrocarbons Alkane Isomerism Conformations of Alkanes IUPAC Nomenclature for Alkanes Classification of Carbon Atoms Branched-Chain Alkyl Groups Cycloalkanes IUPAC Nomenclature for Cycloalkanes Isomerism in Cycloalkanes Sources of Alkanes and Cycloalkanes Physical Properties of Alkanes and Cycloalkanes Chemical Properties of Alkanes and Cycloalkanes Nomenclature and Properties of Halogenated Alkanes Chemical Connections: Chlorofluorocarbons and the Ozone Layer

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

Organic vs Inorganic Compounds

Chapter 12. Saturated Hydrocarbons 12.4 12.6 12.7 12.8 12.10 12.11 12.12 12.13 12.14 12.15 12.16 12.17 12.18

Chapter 12. Saturated Hydrocarbons

12-3

← Fig. 12.1

Sheer numbers is one reason why organic chemistry is a separate field of chemical study.

Chemistry 121, Winter 2009-10, LA Tech

12-4

1

Classification of Hydrocarbons

Types of formula for organic compounds

Fig. 12.2 Terms for organic compounds.

Chemical formula: Indicate the kind and number of each type of atom in the molecule. Condensed formula: Shows skeletal atoms in a molecule and places them in a sequential order that indicates bonding. Structural formula: Shows each atom and bonds in a molecule. Line-angle formula: The hydrogen atoms are removed from carbon chains, leaving just a carbon line skeleton with functional groups attached to it.

Chemistry 121, Winter 2009-10, LA Tech

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Alicyclic Alkanes

Chemistry 121, Winter 2009-10, LA Tech

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Models of Hydrocarbons ← Fig. 12.3

Are saturated hydrocarbons: •Noncyclic alkanes: General molecular formula, CnH2n+2

Molecular structures of (a) methane, (b) ethane, and (c) propane, the three simplest alkanes.

Structural formula:

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

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2

Physical Properties of Hydrocarbons

Organic Nomenclature Organic molecules can be very complex. Naming system must be able to tell

• Number of carbons in the longest chain • The location of any branches • Which functional groups are present and where they are located.

The IUPAC Nomenclature System provides a uniform set of rules that we can follow.

Fig. 12.4 Models of (a) pentane, (b) isopentane, and (c) neopentane.

Chemistry 121, Winter 2009-10, LA Tech

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Naming alkanes

Nomenclature: Unbranched or straight chain alkanes

1 Find the longest carbon chain. Use as base name with an ane ending. 2 Locate any branches on chain. Use base names with a yl ending. 3 For multiple branch of the same type, modify name with di, tri, ... 4 Show the location of each branch with numbers. 5 List multiple branches alphabetically - the di, tri, ... don’t count..

General molecular formula: CnH2n+2 All bond angles about tetrahedral carbon are approximately 109.5°

12-11

Molecular Formula

N ame

Molecular Formula

methane CH4

nonane

C9 H2 0

ethan e C2 H6 propane C H 3 8

decan e

C1 0 H2 2

dodecan e

C1 2 H2 6

tetrad ecane

C1 4 H3 0

N ame

bu tane

C4 H1 0

pen tane C5 H1 2 hexan e

C6 H1 4

hep tane C7 H1 6 octane

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

Chemistry 121, Winter 2009-10, LA Tech

hexadecane C1 6 H3 4 octadecan e

C1 8 H3 8

eicosane

C2 0 H4 2

C8 H1 8 12-12

3

Alkanes

Names of Alkyl Groups

First four members of the alkanes Name Methane

# of C 1

methyl ethyl propyl isopropyl butyl sec-butyl isobutyl tert-butyl

Condensed formula CH4

Ethane

2

CH3CH3

Propane

3

CH3CH2CH3

Butane

4

CH3CH2CH2CH3

Called a homologous series “Members differ by number of CH2 groups” 12-13

Chemistry 121, Winter 2009-10, LA Tech

H4 methane: CH3- methyl CH3CH3 ethane: CH3CH2- ethyl CH3CH2CH2(CH3)2CHCH3CH2CH2CH2CH3CH2 (CH3) CH(CH3)2CHCH2(CH3)3C-

12-14

Chemistry 121, Winter 2009-10, LA Tech

Constitutional isomers in butane Constitutional isomers Compounds with the same number and type of atoms but with different arrangements. Molecular Formula

C5H12

Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms in the skeleton. There are two constitutional isomers with molecular formula C4H10

Condensed structural formulas. CH3CH2CH2CH2CH3 CH3CH(CH3)CH2CH3

(CH3)4C

pentane 2-methylbutane

2,2-dimethylpropane

All are constitutional isomers of C5H12. Chemistry 121, Winter 2009-10, LA Tech

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CH3 CH2 CH2 CH3

CH3 CH3 CHCH3

Butane (bp -0.5°C)

2-Meth ylp ropan e (bp -11.6°C)

Chemistry 121, Winter 2009-10, LA Tech

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4

Molecular Structure and Physical Properties

Alkanes •Bp decreases with hydrocarbon chain branching due to decrease in surface area which results in fewer intermolecular attractions.

Physical Properties

• Mp increases with hydrocarbon chain branching because the more compact molecules have a better fit in the crystal lattice making it more stable.

• Nonpolar molecules • Not soluble in water

•Solubility - the quantity of solute that will dissolve in a solvent depends on polarity of solute and solvent. “Like dissolves like” refers to polar liquids tending to dissolve polar solutes and nonpolar liquids tend to dissolve nonpolar solutes. Alkanes are nonpolar. 12-17

Chemistry 121, Winter 2009-10, LA Tech

Chemistry 121, Winter 2009-10, LA Tech

bp, oC -161.7 - 88.6 - 42.2 -0.5 36.1 68.7 98.4 125.6 150.7 174.0

mp, oC -182.6 -182.8 -187.1 -135.0 -129.7 - 94.0 - 90.5 - 56.8 -53.7 -29.7

• Low density • Low melting point

• Low boiling point 12-18

Chemistry 121, Winter 2009-10, LA Tech

Physical Properties of The Saturated Hydrocarbons

Physical Properties of Alkanes Name Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane

These go up as the number of carbons increases.

Density at 20 oC 0.000667 0.00125 0.00183 0.00242 0.626 0.659 0.684 0.703 0.718 0.730 12-19

Name

Molecular Formula

Melting Point (oC)

Boiling Point (oC)

State at 25oC

methane

CH4

-182.5

-164

gas

ethane

C2H6

-183.3

-88.6

gas

propane

C3H8

-189.7

-42.1

gas

butane

C4H10

-138.4

-0.5

gas

pentane

C5H12

-129.7

36.1

liquid

hexane

C6H14

-95

68.9

liquid

heptane

C7H16

-90.6

98.4

liquid

octane

C8H18

-56.8

124.7

liquid

nonane

C9H20

-51

150.8

liquid

decane

C10H22

-29.7

174.1

liquid

eicosane

C20H42

36.8

343

Chemistry 121, Winter 2009-10, LA Tech

solid

12-20

5

Refining Crude Oil

Sources of Alkanes

→ Fig. 12.9 The complex hydrocarbon mixture present in petroleum

is separated into simpler mixtures by means of a fractionating column.

Natural gas 90-95% methane, 5-10% ethane

Petroleum • • • • • •

gases (bp below 20°C) naphthas, including gasoline (bp 20 - 200°C) kerosene (bp 175 - 275°C) fuel oil (bp 250 - 400°C) lubricating oils (bp above 350°C) asphalt (residue after distillation)

Coal Chemistry 121, Winter 2009-10, LA Tech

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Rules of IUPAC Nomenclature of Branched Alkanes Parent name: the longest carbon chain Substituent: a group bonded to the parent chain Alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol Rwritten in alphabetical order CH4 becomes CH3- (methyl) CH3CH3 becomes CH3CH2- (ethyl)

Prefixes: di-, tri-, tetra-, etc. are not included in alphabetization Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

IUPAC Nomenclature of branched alkanes • suffix -ane specifies an alkane • prefix tells the number of carbon atoms

Prefix Carbons meth1 eth 2 prop3 but4 pent5 hex6 hept7 oct8 non 9 dec10 Chemistry 121, Winter 2009-10, LA Tech

Carb on s Prefix undec11 dod ec12 tridec13 tetradec14 pentadec- 15 hexadec16 heptadec- 17 octad ec18 non adec19 eicos 20 12-24

6

Alkyl groups and their IUPAC names

Example

Fig. 12.5 The four most common branched-chain alkyl groups and their IUPAC names.

Name the following. (CH 3)2CHCH2CH2CH(CH3)2 This is a condensed structural formula. First convert it to a carbon skeleton, leaving out the hydrogen.

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Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

Common alkyl groups N ame methyl

Con dens ed Structu ral Formula -CH3

ethyl

-CH2 CH3

propyl

-CH2 CH2 CH3

isopropyl -CHCH3 CH3 bu tyl

-CH2 CH2 CH2 CH3

N ame isobu tyl sec-butyl

Con dens ed Structu ral Formula -CH2 CHCH3 CH3

(CH 3)2CHCH2CH2CH(CH3)2

-CHCH2 CH3 CH3

C C | | C-C-C-C-C-C

CH3 t ert-bu tyl -CCH3 CH3

Now name it! Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

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7

Giving IUPAC names • CH3CH2CH2CH(CH3)CH2CH2CH2CH3

Parent name: octane

C C | | C-C-C-C-C-C

subs tituent

parent chain

Substituent: Methyl at 4 4-mehtyl 1

Name: 4-Methyloctane

1. Longest chain is 6 - hexane

2

3

4

5

6

8 7

4-Methyloctane

CH3C(CH3)2CH2CH(CH2CH3)CH2CH3

2. Two methyl groups - dimethyl CH3 CH2 CH3

3. Use 2,5-dimethylhexane

CH3 CCH2 CHCH2 CH3 CH3

1

2

3

4

6 5

4-Ethyl-2,2-dimeth ylh exane 12-29

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

Physical State of Saturated Hydrocarbons

Types of carbon and hydrogen atoms Primary (1°) carbon: a C bonded to one other carbon Secondary (2°) carbon : a C bonded to two other carbons Tertiary (3°) carbon : a C bonded to three other carbons Quaternary (4°) carbon : a C bonded to four other carbons

a 2° carb on a 4° carb on a 3° carb on CH3 CH3 -C-CH2 -CH-CH3 a 1° carb on a 1° carb on CH3 CH3

← Fig. 12.12

A physical-state summary for unbranched alkanes and unsubstituted cycloalkanes at room temperature and pressure.

2,2,4-Trimeth ylp entane Primary (1°) hydrogen: Secondary (2°) hydrogen : Tertiary (3°) hydrogen :

Chemistry 121, Winter 2009-10, LA Tech

a H bonded to primary (1°) carbon a H bonded to secondary (2°) carbon a H bonded to tertiary (3°) carbon

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8

Reactions of alkanes

Reactions of alkanes Halogenation • A reaction where a halogen replaces one or more hydrogens.

CH4(g) + Cl2(g)

heat or light

CH3Cl(g) + HCl(g)

Used to prepare many solvents

• dichloromethane - paint stripper • chloroform - once used as anesthesia • 1,2-dichloroethane - dry cleaning fluid

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Chemistry 121, Winter 2009-10, LA Tech

Reactions of Alkanes

heat of combustion: heat released when one mole of a substance is oxidized to carbon dioxide and water in a combustion reaction.

CH3 CH2 CH3 + 5O2 Propane

CO2 + 2 H2 O

CH4(g) + 2O2(g)

Many alkanes are used this way - as fuels • • • •

Methane Propane Butane Gasoline

-

natural gas used in gas grills lighters mixture of many hydrocarbons,

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Chemistry 121, Winter 2009-10, LA Tech

H° = -212 kcal/mol

A mixture of carbon monoxide and hydrogen in varying proportions, depending on how it is produced C + H2 O Coal CH4 + 1 O2 2 Methan e

CO + 2 H2

3CO2 + 4H2 O H° = -530 kcal/mol

12-35

heat catalyst

CO + H2 CO + 2 H2

Methanol and acetic acid are produced from synthesis gas

CH3 OH + CO Methan ol Chemistry 121, Winter 2009-10, LA Tech

CO2(g) + 2H2O(g)

Synthesis Gas

Oxidation is the basis for the use of alkanes as energy sources for heat and power

CH4 + 2O2 Methane

Combustion

Chemistry 121, Winter 2009-10, LA Tech

catalyst

catalyst

CH3 OH Methan ol O CH3 COH Acetic acid

12-36

9

Cycloalknes

Cycloalkanes

Cyclic alkanes: General molecular formula, CnH2n Structure and nomenclature • named similar to noncyclic alkanes • to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring • if only one substituent, no need to give it a number • if two substituents, number from the substituent of lower alphabetical order • if three or more substituents, number to give them the lowest set of numbers, and then list substituents in alphabetical order • in planar cyclopentane, all C-C-C bond angles are 108°, which differ only slightly from the tetrahedral angle of 109.5°consequently there is little angle strain

Fig. 12.6 Simple cycloalkanes

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Chemistry 121, Winter 2009-10, LA Tech

Properties of Hydrocarbons cont’d

Chemistry 121, Winter 2009-10, LA Tech

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Melting Points of Hydrocarbons

CAG 12.1

→ Fig. 12.11

For a series of alkanes or cycloalkanes, melting point increases as carbon chain length increases.

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

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10

Naming simple of cycloalkanes:

Ring strain in cycloalkane

cyclopropane

The stability of cycloalkanes depends on their ability to relieve ring strain when the bond angles are less than 109.5˚.

cyclobutane cyclopentane cyclohexane

60º

90º

108º

cycloheptane

109.5º

Most stable

Least stable

cyclooctane 12-41

Chemistry 121, Winter 2009-10, LA Tech

Naming branched cycloalkanes Commonly written as line-angle formulas

12-42

Chemistry 121, Winter 2009-10, LA Tech

Cycloalkanes :saturated hydrocarbons with a carbon ring

examples:

Have C-C single bonds in a ring structure. General formula CnH2n Isopropylcyclopentan e

1-tert -Bu tyl-4-methylcycloh exane

1-Isobutyl-2-meth ylcyclohexan e

1-Ethyl-1-methylcyclopropane

cyclopropane

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

cyclobutane

12-44

11

Naming Cycloalkanes

Conformations of Cycloalkanes

Have the carbons connected in a ring. These compounds are known collectively as To name a cycloalkane, use the prefix cyclo- with the parent. If there is only one substituent, a number is not needed.

Cyclohexane Chair conformation-low energy Boat conformation-higher energy

Chemistry 121, Winter 2009-10, LA Tech

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Geometrical (cis & trans) Isomers of Cycloalkanes

12-46

Cis and trans Geometrical isomers of Cycloalkanes

Carbon ring create a rigid structure trans and cis is used to describe the arrangements of alkyl groups with respect to the plane of the ring cis: on the same side trans: on the opposite sides

Chemistry 121, Winter 2009-10, LA Tech

Chemistry 121, Winter 2009-10, LA Tech

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two groups are said to be located cis to each other if they lie on the same side of a plane. If they are on opposite sides, their relative position is described as trans.

Chemistry 121, Winter 2009-10, LA Tech

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12

Conformations of cyclopentane

Boat conformations of cyclohexane

• In planar cyclopentane, there are 10 fully eclipsed C-H bonds, which create torsional strain of approximately 10 kcal/mol • Puckering to an “envelope” conformation relieves part of this strain • In an envelope conformation, eclipsed interactions are reduced but angle strain is increased slightly (105°)

Planar conformation Chemistry 121, Winter 2009-10, LA Tech

Envelope conformation 12-49

A puckered conformation in which carbons 1 and 4 are bent toward each other • a boat conformation is less stable than a chair conformation by 6.5 kcal (27 kJ)/mol • torsional strain is created by four sets of eclipsed hydrogen interactions • steric strain (nonbonded interaction strain) is created by one set of flagpole interactions

Chemistry 121, Winter 2009-10, LA Tech

12-50

Interconversions of conformations Interconvert ion to alternative chair conformations (lower energy) via a boat conformation (higher energy

Geometric isomerism: A stereoisomer concerning the orientation differences of atoms or groups around a double bond or ring cis isomer - a geometric isomer where groups are on the same side of a ring or double bond

flip th is end dow n

trans isomer - a geometric isomer where groups are on opposite sides of a ring or double bond

flip th is end up

Chemistry 121, Winter 2009-10, LA Tech

Cis and Trans Geometric Isomerism in Cycloalkanes

12-51

Chemistry 121, Winter 2009-10, LA Tech

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13

CH 3

cis-1,2-dimethylcyclopentane

CH 3

Problem: draw the alternative chair conformations of this trisubstituted cyclohexane and state which is the more stable

CH 3

trans-1,2-dimethylcyclopentane

H3 C CH 3

cis-1,4-dimethylcyclohexane, equ atorial H CH3 H axial CH3

Perspective drawing

H

CH3 CH3

H

H H

CH3 H

axial

CH3

cis-1,4-D imethylcycloh exane (th ese conformation s are of equ al stability) Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

12-54

CFCs and Ozone Depletion

Substitution Reactions of Alkanes → Fig. 12.14 In an alkane substitute reaction, an incoming molecule.

atom or group of atoms replaces a hydrogen atom in the alkane

 CC 12. 3

Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

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14

Halo-Alkanes

Saturated Hydrocarbons Derivatives

Fig. 12.15 Models of four ethyl halides.

Table 12.4 Chemistry 121, Winter 2009-10, LA Tech

12-57

Isomerism

Constitutional Isomers Skeletal (Chp. 12)

Isomers - different compounds having the same molecular formula but different structural formulas There are two type of Isomers: Constitutional isomerism: Different connections among atoms in the • Skeleton • Position • Functional group Stereoisomerism: Same connectivity among atoms, but these atoms differ in spatial orientation • geometric • conformational • optical Chemistry 121, Winter 2009-10, LA Tech

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Chemistry 121, Winter 2009-10, LA Tech

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CH 3 CH 3CH 2CH 2CH 3

CH 3CHCH 3

Functional (Chp. 14)

Positional (Chp. 12)

CH 3CH 2OH CH 3OCH 3

CH 3CH 2CH 2 CH 3CH CH 3 Br Br

Stereoisomers Geometric (Chp. 13) Conformational (Chp.12) H H C C Br Br

H Br C C Br H

H H

CH 3 CH 3

CH 3 CH 3 H

H

HH

HH

Enantiomers (Optical) (Chp. 16) H Chemistry 121, Winter 2009-10, LA Tech

CO 2H C OH CH 3

HO

CO 2H C H CH 3

12-60

15

Physical Properties Constitutional isomers

Constitutional isomerism in alkane The number of constitutional isomerism increases with the carbon number in the alkane

Mole cular Formula CH 4

Constitutional isomers are different compounds and have different physical properties

Constitutional Is ome rs

C5 H1 2

1 3

C1 0 H2 2

75

C1 5 H3 2

4,347

N ame hexan e 3-methylp entane 2-methylp entane

Meltin g Boiling Point Point D en sity (°C) (°C) (g/mL) -95

69

0.659

64 62 58

0.664 0.653 0.662

50

0.649

C2 5 H5 2

36,797,588

-6 -23 2,3-dimethylbutan e -129

C3 0 H6 2

4,111,846,763

2,2-dimethylbutan e -100

Chemistry 121, Winter 2009-10, LA Tech

Hexane

2,2-Dimethylbutane 12-61

Chemistry 121, Winter 2009-10, LA Tech

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16