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2009년도 제2학기 화 학 2 담당교수: 신국조 Textbook: P. Atkins / L. Jones, Chemical Principles, 4th ed., Freeman (2008) Chapter 18

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CHAPTER 18 ORGANIC CHEMISTRY I: THE HYDROCARBONS

◈ Hydrocarbons ◆ Aliphatic hydrocarbons: ◆ Aromatic hydrocarbons:

Chains of carbon atoms benzene ring

ALIPHATIC HYDROCARBONS

18.1 Types of Aliphatic Hydrocarbons

◆ Saturated aliphatic hydrocarbons: Alkanes

No multiple carbon-carbon bonds

Consist of tetrahedral arrangements of bonds at each carbon atom

Rotation about C – C bond axis (ball shape ↔ zigzag chain)

▶ Cycloalkanes : Single-bonded ring of carbon atoms


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▷ Isomers : butane vs. methylpropane

▷ Condensed structural formula: showing how the atoms are grouped together

Butane: CH3CH2CH2CH3 CH⎯⎯→ 3(CH2)2CH3

Methylpropane: CH3CH(CH3)CH3 (CH⎯⎯→ 3)3CH

▷ Line structure: representing a chain of carbon atoms in a zigzag line

Self-Test 18.1A Draw (a) the line structure of aspirin and (b) the structural formula for CH3(CH2)2C(CH3)2CH2C(CH3)3.


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(a) Acetylsalicylic acid (aspirin)

⎯⎯→

(b) CH3(CH2)2C(CH3)2CH2C(CH3)3

⎯⎯→

Self-Test 18.1B Write (a) the structural formula of carvone and (b) the condensed structural formula for H3C – CH – CH2 – CH2 – CH2 – CH – CH2 – CH3 .

| | CH3 CH3

(a) Carvone

⎯⎯→

(b) H3C–CH–CH2 –CH2 –CH2 –CH –CH2 –CH3

| | CH3 CH3

⎯⎯→ (CH3)2CH(CH2)3CH(CH3)CH2CH3

◆ Unsaturated aliphatic hydrocarbons: Alkenes & Alkynes

One or more multiple carbon-carbon bonds

▶ Alkenes: double bonds

(ethylene)


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▶ Alkynes: triple bonds

(acetylene)

TOOLBOX 18.1 HOW TO NAME ALIPHATIC HYDROCARBONS

◈ Alkanes: ▶ Straight-chain alkanes: -ane Methane, ethane, propane, butane, pentane, …… ▶ Side chain: -yl ex: CH3CH2– ethyl group ▶ Branched-chain alkanes : based on the longest continuous carbon chain ex: 2-methylbutane CH3 – CHCH3

| CH2CH3

▶ Cyclic alkanes: cyclo- ex: cyclopropane

▶ Numbering the position of a branch or substituent Longest chain Consecutive numbering leading to the lower number(s) to the substituent(s)

ex: 3 2 2 2 2

2 3

3C H C H C H C H C H C H | CH CH

3C H C(CH ) C H C H

H C CH | |

H C C C C H | | H C H

− − −

6 5 4 3 2 11 2 3 4

1 2 3 4

3 3 2 2

3-ethylhexane 2,2-dimethylbutane ▶ Number of substituents: prefixes di-, tri-, tetra-, penta-, hexa-,… Listed in alphabetical order Numbers with commas followed by “-” indicate carbons to which substituents are attached.

3 3

3 3

3

2,2,3-trimethylbutane

1-ethyl-2-methylcyclopentane ◈ Alkenes and Alkynes: ▶ Double bond: -ane -ene ▶ Triple bond: -ane -yne ▶ Position of multiple bond: Number of the first (lower-numbered) carbon atom involved ▶ More than one multiple bonds: Greek prefix ▶ Priority of numbering: functional group double bond triple bond prefixed group


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CH3 – CH2 – CH2 – C ≡ CH 1-pentyne CH3 – CH2 – CH = CH – CH3 2-pentene CH2 = CH – CH2 – CH = CH2 1,4-pentadiene

Ex. 18.1 Naming alkanes and cycloalkanes (a) Name the following compound:

Count the number of carbon atoms in the longest chain.

The longest chain has 5 C atoms. substituted pentane

Identify and count substituents. 3 methyl groups (CH3 – )

trimethylpentane

Numbering of backbone C atoms to give the lowest numbers to the substituents

2,2,4-trimethylpentane

(b) Write the structural formula of 2-ethyl-1,1-dimethylcyclohexane Draw the longest chain of carbon atoms.

Cyclohexane Ring of 6 carbon atoms

Number the carbon atoms and add the substituents according to the number given in the name.

Add 2 methyl groups to carbon atom 1 and add an ethyl group to carbon 2.

Add hydrogen atom to give each C atom a valence of four.


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Self-Test 18-2A (a) Name the following compound:

4-ethyl-3-methyloctane

(b) Write the structural formula of 5-ethyl-2,2-dimethyloctane.

Ex. 18.2 Naming alkenes (a) Name the alkene CH3CH2CH=CH2. 1-butene (b) Write the condensed structural formula for 5-methyl-1,3-hexadiene. CH2=CHCH=CHCH(CH3)CH3 18-2 Isomers

Fig. 18.1 Isomerisms in organic chemistry

◈ Structural isomers Same atoms, Different connectivity Ex: Insertion of – CH2 – group in two different ways for two isomers of C4H10 formula.


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Ex. 18.3 Writing the formulas of structural isomers Draw two-dimensional structural formulas for all the isomeric alkanes of formula C5H12.

[Solution] Insert – CH2 – group into different parts of two isomers with the formula C4H10. (1) From butane,

(2) From methylpropane

Notice that (b) ≡ (c) ≡ (e). There are only three distinct isomers, (a), (b), and (d). QED.

◈ Stereoisomers

Same connectivity, Different spatial arrangements of atoms

▶ Geometrical isomers

(a) different arrangements on either side of a double bond

(b) different arrangements above or below the ring of a cycloalkane

2-butene

Fig. 18.2 Two pairs of geometrical isomers.


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Self-Test 18.5A

trans-2-pentene cis-2-pentene cis-propenylbenzene trans-propenylbenzene

▶ Optical Isomers

● each other’s nonsuperimposable mirror images

● chiral molecules

● enantiomers: a chiral molecule and its mirror image

● chiral carbon atom: to which four different groups are attached

● achiral molecule

Fig. 18.3 3-methylhexane and its mirror image. achiral molecule

● Enantiomers have identical chemical properties,

except when they react with other chiral molecules

different odors and different pharmacological activities

● Enantiomers differ in one physical property:

optical activity: rotating the plane of polarized light

● Organic compounds

Racemic mixtures from laboratory synthesis

Only one enantiomer from reactions in living cells!

Ex. 18.4 Deciding whether a compound is chiral.

Decide whether (1) 2-bromobutane and (2) 2-bromopropane are chiral.


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CH3CH2C*HBrCH3 CH3CHBrCH3

chiral carbon achiral

18-3 Properties of Alkanes Electronegativity: C(2.6), H(2.2)

Nonpolar interaction: London force

Insoluble in water

Increases with number of electrons

Increase in chain length

Less volatile

C1 ~ C4 : gases

C5 : volatile liquid

C6 ~ C11 : moderately volatile liquids

m.p., b.p., △Hvap

Unbranched alkanes > branched isomer

due to different packings

Fig. 18.4 The melting and boiling points of unbranched alkanes.

Fig. 18.5 The straight chain (a) and branched alkanes (b).

“Paraffins” (L. “little affinity”) – Not very reactive: against conc H2SO4, HNO3, KMnO4, boiling NaOH(aq) – Strong bond enthalpy (in kJ∙mol–1): C-C (348), C-H (412) C=O (743), C-O (360), C-F (484), O-H (463), O=O (496) – Undergo oxidation and substitution – Used as fuels

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g), ∆H = – 890 kJ


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18-4 Alkane Substitution Reactions ◈ Substitution reaction (치환반응,置換反應): Replacement of an atom (in alkanes, an H atom) by another atom or a group of atoms leads to functionalization of alkanes

Fig. 18.6 An alkane substitution reaction.

▶ Halogenation of alkanes: Radical chain reaction

CH4(g) + Cl2(g) CH or hν ∆⎯⎯⎯⎯→ 3Cl(g) + HCl(g)

Initiation: Cl2 Cl∙ + ∙Cl or hν ∆⎯⎯⎯⎯→Propagation: Cl∙ + CH4 HCl + ∙CH⎯⎯→ 3

Cl2 + ∙CH3 CH⎯⎯→ 3Cl + Cl∙

Termination: Cl∙ + ∙Cl Cl⎯⎯→ 2

18-5 Properties of Alkenes

▶ C=C double bond : more reactive than C–C single bond

One σ -bond between sp2 hybrid orbitals of C atoms

One π -bond from overlap of p orbitals

No rotation about C=C bond axis

▶ No compact packing low m.p.

▶ Produced during the refining of petroleum:

Fig. 18.7 The π -bond in an alkene.

◈ Elimination reaction (1): CH3CH3(g) CH2 3Cr O⎯⎯⎯→ 2=CH2(g) + H2(g) dehydrogenation (제거반응,除去反應)

Fig. 18.8 An elimination reaction


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◈ Elimination reaction (2): dehydrohalogenation of haloalkanes

CH3CH2CHBrCH3 + CH3CH2O– CHoEtOH at 70 C⎯⎯⎯⎯⎯⎯→ 3CH=CHCH3 + CH3CH2OH + Br–

ethoxide

⎯⎯→

18-6 Electrophilic Addition (친전자성 첨가반응, 親電子性添加反應) ◈ Addition reaction

Halogenation: CH2=CH2 + Cl2 CH⎯⎯→ 2Cl – CH2Cl Hydrohalogenation: CH2=CH2 + HCl CH⎯⎯→ 3 – CH2Cl

Fig. 18.9 An addition reaction.

Fig. 18.10 The electrostatic potential diagrams of ethane (a) and ethene (b).

The electron-rich red region around the double bonds in ethane.

◆ Bromination of ethene: H2C=CH2 + Br2 1,2-dibromoethane ⎯⎯→(1) Approach of Br2 toward ethene separation of charges in Br2 (becomes an electrophile) (2) Formation of a cyclic “bromonium ion” after the Br–Br bond breaks.

Fig. 18.11 The approach of Br2

toward the double bond of ethene

⎯⎯→


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(3) Immediate attack of an incoming Br – ion breaking the existing C(1) – Br bond while forming C(1)–Br and C(2)–Br bonds.

⎯⎯→

◆ Hydrogenation of alkene

2catalystH ( ) C C CH CHg + ⋅⋅ ⋅ = ⋅ ⋅ ⋅ ⎯⎯⎯⎯→ ⋅⋅⋅ − ⋅ ⋅ ⋅

rigid flexible compact packing

Fig. 18.12 Shortening of a fluid oil into a solid fat.

AROMATIC COMPOUNDS, Arenes or Aryl groups 18-7 Nomenclature of Arenes

(ortho-dinitrobenzene) (meta-dinitrobenzene) (para-dinitrobenzene) (o-dinitrobenzene) (m-dinitrobenzene) (p-dinitrobenzene) Ex. 18.5 Naming aromatic compounds

1-ethyl-3-methylbenzene 1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene (o-xylene) (m-xylene) (p-xylene)


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Self-Test 18.9A Self-Test 18.9B

1-ethyl-3-propylbenzene 1-ethyl-3,5-dimethyl-2-propylbenzene

18-8 Electrophilic Substitution

Stability of de-localized π -electrons in the benzene ring

substitution rather than addition to the double bonds

◆ Electrophilic substitution (친전자성 치환반응) ▶ Bromination of benzene:

6 6 2 6 53FeBrC H Br C H Br HBr+ ⎯⎯⎯→ +

(1) Catalyst acts as a Lewis acid to form a complex.

3 3: Br Br : FeBr : Br Br FeBrδ δ

Fig. 18.13 The catalyst FeBr3 forming

a complex with a Br2 molecule. +− + ⎯⎯→ − − −

(2) Attack of an electrophile on electron-rich ring forming a positively charged intermediate

⎯⎯→ + :Br-FeBr3–

(3) H atom pulled from the ring by :Br-FeBr3– complex

due to the stability of π -electrons in the ring, forming HBr and releasing FeBr3.

⎯⎯→ + HBr + FeBr3

▶ Nitration of benzene

3 2 4 2 4 2HNO H SO NO HSO H O+ −+ ⎯⎯→ + +

⎯⎯→ ⎯⎯→

+ H+

H pulled from the ring by HSO4– ion


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▶ Electron-donating substituents: –OH, –NH2

Electrophilic substitution proceeds rapidly!

Ex. Nitration of phenol proceeds rapidly without a catalyst.

Only products are ortho- and para-nitrophenols. Why?

Fig. 18.14 The electron distributions in benzene and phenol. ▷ Resonance hybrid structures of phenol: Electron-rich in ortho- and para- position

←⎯→

←⎯→ ←⎯→

☻ Ortho-, para-directing substituents: –OH, –NH2, –Cl, –Br

▶ Electron-withdrawing substituents: carboxyl group (–COOH)

highly electronegative substituents

withdrawing electrons by resonace

slowing down electrophilic substitutions

Ex. Nitration of benzoic acid (C6H5COOH): slow, meta- product dominant !

Benzoic acid

⎯⎯→

18.5%

+

80.0%

+

1.5%


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▷ Resonance hybrid structures of benzoic acid: Electron-poor in ortho- and para- position Electron-rich in meta- position

←⎯→

←⎯→

←⎯→

☻ Meta-directing substituents: –COOH, –NO2, –CF3, –C≡N IMPACT ON MATERIALS: FUELS

Source of hydrocarbons: fossil fuels (petroleum and coal)

Aliphatic hydrocarbons from petroleum (mixture of aliphatic and aromatic hydrocarbons) Aromatic hydrocarbons from petroleum and coal Kerosene; C10~C16

fuel for jet and diesel engines Lubricating oils; C17~C22

paraffin wax and asphalt Gasoline; C5~C11

Fig. 18.15 A platform for extracting petroleum from beneath the ocean.

18-9 Gasoline

▶ Separation of hydrocarbons in petroleum by fractional distillation

▶ Cracking:

16 34 8 16 8 18, catalystC H C H C H∆⎯⎯⎯⎯→ +


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▶ Alkylation:

4 10 4 8 8 18catalystC H C H C H+ ⎯⎯⎯⎯→

▶ Isomerization:

33 2 6 3 3 3 2 2 3 3

AlClCH (CH ) CH CH C(CH ) CH CH(CH )CH⎯⎯⎯→

▶ Aromatization:

2 23 3,3 2 5 3 3 6

Al O Cr OCH (CH ) CH CH C H⎯⎯⎯⎯⎯→

18-10 Coal

Fig. 18.16 A schematic representation of a part of the structure of coal.

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