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New Way Chemistry for Hong Kong A-Level Book 3A11

Introduction to Introduction to Organic ChemistryOrganic Chemistry

21.121.1 What is Organic Chemistry?What is Organic Chemistry?21.221.2 The Unique Nature of CarbonThe Unique Nature of Carbon21.321.3 Classification of Organic CompoundsClassification of Organic Compounds21.421.4 Factors Affecting the Physical Properties of Factors Affecting the Physical Properties of

Organic CompoundsOrganic Compounds

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New Way Chemistry for Hong Kong A-Level Book 3A2New Way Chemistry for Hong Kong A-Level

3A2

21.21.11 What is What is

Organic Organic Chemistry?Chemistry?


3A3

21.1 What is Organic Chemistry (SB p.2)

Organic ChemistryOrganic Chemistry• Chemistry of the compounds present in living organisms.• They all contain carbon.• Organic Chemistry is the Chemistry of Carbon.


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Living things

Carbohydrates / Proteins / Fats / Vitamins / Antibiotics

21.1 What is Organic Chemistry (SB p.4)Natural Sources of Organic Natural Sources of Organic CompoundsCompounds

A variety of organic products

obtained from living things


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Crude oil or coal

Fractional distillation / destructive distillation

Alkanes / Alkenes / Alkynes / Aromatic hydrocarbons

21.1 What is Organic Chemistry (SB p.4)Natural Sources of Organic Natural Sources of Organic CompoundsCompounds

A variety of useful products

derived from crude oil and

coal

Check Point 21-1Check Point 21-1


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In the past …,

Chemistry

Organic compound

sobtained from living

organisms

Inorganic compound

sobtained from non-living sources


Development of Organic Development of Organic Chemistry as a ScienceChemistry as a Science


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(Inorganic compound)

(Organic compound)

In 1828, Wohler (a German chemist)




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Redefining … ...Organic chemistry is the study of carbon compounds (except CO, CO2, carbonates, hydrogencarbonates, carbides and cyanides) obtained from natural sources or synthesized in the laboratories.




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21.21.22 The Unique The Unique

Nature of Nature of CarbonCarbon


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21.2 The Unique Nature of Carbon (SB p.5)

Ability to form four Ability to form four strongstrong covalent bonds covalent bonds

Carbon (ground state)

• Electronic configuration of carbon (ground state) : 1s22s22p2


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• Each carbon atom has four unpaired electrons when excited

• Tend to form four strong covalent bonds

Carbon (excited state)

Ability to form four Ability to form four strongstrong covalent bonds covalent bonds


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• Carbon atoms link together to form chains of varying length, branched chains and rings of different sizes

• Catenation: Ability of atoms in forming stable

bonds with itself, hence joining up into chains or rings

Ability to Ability to CatenateCatenate


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Ability to Ability to CatenateCatenateC – C > Si – Si > Ge – Ge > Sn – SnBond strength as bond length


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Ability to Ability to CatenateCatenateC – C > N – N > O – O

Bond strength as the number of lone pairs


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Ability to Ability to CatenateCatenateCnH2n+2 n = 1,2,3,…(no limit for n)SinH2n+2 n = 1 to 6 only silanesGenH2n+2 n = 1 to 3 only germanesSnnH2n+2 Only SnH4 (stannane) exists


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Carbon (excited state)


Ability to Form Multiple Ability to Form Multiple BondsBonds

sp2 bonds, 2 bonds

sp2

1 bond, 3 bonds

sp3

4 bonds


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Single bond Double bond Triple bond


* X = halogens


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Example 21-2Example 21-2 Check Point 21-2Check Point 21-2


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21.21.33Classification Classification

of Organic of Organic CompoundsCompounds


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• Organic compounds are classified by the the presence of characteristic functional groups.

21.3 Classification of Organic Compounds (SB p.7)

Functional Functional GroupsGroups


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A functional group is defined as an atom or a group of atoms that effectively determines the chemical properties of an organic compound.




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Functional Functional GroupsGroups• Propane does not react with

sodium• Ethanol and propan-1-ol react

with sodium to give hydrogen gas


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• have similar chemical properties they contain the same functional group –OH they are classified into the same homologous series — alcohols

and




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Homologous SeriesHomologous Series21.3 Classification of Organic Compounds (SB p.12)

A homologous series is a series of compounds that have the same functional group, and each member differs from the next member by a – CH2 – unit in their formulae.

CH4 C2H6 C3H8 C4H10

CH2 CH2 CH2


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Number of carbon atom(s)

IUPAC name

Molecular formula

Condensed structural formula

Structural formula

1 Methane CH4 CH4

2 Ethane C2H6 CH3CH3

3 Propane C3H8 CH3CH2CH3

4 Butane C4H10 CH3CH2CH2CH3

The first four members of straight-chain alkanes


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Number of carbon atom(s)

IUPAC name

Molecular formula

Condensed structural formula

Structural formula

1 Methanol CH3OH CH3OH

2 Ethanol C2H5OH CH3CH2OH

3 Propan-1-ol

C3H7OH CH3CH2CH2OH

4 Butan-1-ol

C4H9OH CH3CH2CH2CH2OH

The first four members of straight-chain alcohols


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• Members in the same series can be represented by a general formula.

e.g. alkanes: CnH2n+2

alkenes: CnH2n

alkynes: CnH2n-2


Homologous SeriesHomologous Series


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• Members in the same series can be represented by a general formula.



e.g. alkanols: CnH2n+1OH

alkanals: CnH2n+1CHO

alkanoic acids: CnH2n+1COOH


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Functional group of an

organic compound

Chemical properties

Members of a homologous series have similar chemical

properties




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• The physical properties change gradually along the homologous series

• e.g. the longer the carbon chain in the molecule ( or the greater the

molecular mass) the greater the attractive force

between molecules the higher the melting point,

boiling point and density




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Number of

carbon atom(s)

Molecular formula

State (at room

temperature and

pressure)

Melting point (°C)

Boiling point (°C)

Density of solid / liquid at 20°C (g cm–3)

123456789

10

CH4

C2H6

C3H8

C4H10

C5H12

C6H14

C7H16

C8H18

C9H20

C10H22

GasGasGasGas

LiquidLiquidLiquidLiquid Liquid Liquid

–183–172–188–135–130

–95–91–57–54–30

–161–89–42

0366998

126151174

––––

0.6260.6570.6840.7030.7180.730

Some physical properties of the first 20 members of straight-chain alkanes



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Number of

carbon atom(s)

Molecular formula

State (at room

temperature and

pressure)

Melting point (°C)

Boiling point (°C)

Density of solid / liquid at 20°C (g cm–3)

11121314151617181920

C11H24

C12H26

C13H28

C14H30

C15H32

C16H34

C17H36

C18H38

C19H40

C20H42

LiquidLiquidLiquidLiquidLiquidLiquidLiquid SolidSolidSolid

–26–10

–7–3101822283237

196216233260271287302316330344

0.7400.7490.7530.7610.7690.7730.7780.7770.7770.785

Some physical properties of the first 20 members of straight-chain alkanes



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Example 21-3AExample 21-3A Example 21-3BExample 21-3B

Example 21-3CExample 21-3C Check Point 21-3Check Point 21-3


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21.21.44Factors Affecting Factors Affecting the Physical the Physical Properties of Properties of

Organic Organic CompoundsCompounds

Refer to notes on ‘Bonding and Structure’ pp.77-92 – intermolecular forces


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1. Structure of the functional group1.1 Dipole moment of the molecule1.2 Formation of hydrogen bonding

2. Length of carbon chains (London dispersion forces)

21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17)

Main Factors Affecting the Main Factors Affecting the Physical Properties of Physical Properties of Organic CompoundsOrganic Compounds


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• Molecules having a polar functional group have a higher b.p. than others with a non-polar functional group of similar molecular masses Stronger intermolecular attraction among molecules


Structure of Functional Structure of Functional GroupGroup


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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17)Structure of Functional Structure of Functional

GroupGroupMolecule Relative

molecular mass

Boiling point (oC)

Molecules with polar functional

groups

CH3CH2CH2OH 60 97.2

CH3CH2CH2NH2 59 48.6

CH3CH2Cl 64.5 12.5

CH3CH2COOH 60 141

Molecules with non-

polar functional

groups

CH3CH2CH2CH3 58 -0.5

CH3CH2CH=CH2 56 -6.2

CH3CH2CCH 54 8.1


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Dipole Moment of MoleculeDipole Moment of Molecule• Tetrachloromethane has 4 polar

bonds in the molecule• M.p. and b.p. are very low

the molecule is non-polar the molecule is

tetrahedrally symmetrical the dipole moments of the

C Cl bond cancel each other


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Examples of Polar Molecules Examples of Polar Molecules with Net Dipole Momentwith Net Dipole Moment


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Examples of Non-polar Examples of Non-polar Molecules with No Net Dipole Molecules with No Net Dipole MomentMoment


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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)Solubility of Organic Solubility of Organic

MoleculesMolecules• Depends on the polarity of organic

molecules and the solvent• Non-polar or weakly polar

compounds dissolve readily in non-polar or weakly polar solvents

• Highly polar compounds dissolve readily in highly polar solvents

• “Like dissolves like”


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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)Solubility of Organic Solubility of Organic

MoleculesMolecules

Hexane in tetrachloromethane

Hexane in water


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Why does Hexane Dissolve Readily Why does Hexane Dissolve Readily in Tetrachloromethane?in Tetrachloromethane?

Intermolecular forces among hexane molecules and those among tetrachloromethane

molecules

Intermolecular forces between hexane and tetrachloromethane

molecules


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Why is Hexane Insoluble in Why is Hexane Insoluble in Water?Water?


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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.19)Formation of Hydrogen Formation of Hydrogen

BondingBonding• Molecules having OH or NH2

groups are able to form hydrogen bonds

• Hydrogen bonds affect the physical properties of alcohols and amines with low molecular masses


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Why does Propan-1-ol have Why does Propan-1-ol have a Higher Boiling Point?a Higher Boiling Point?


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Formation of Hydrogen Formation of Hydrogen BondingBonding• Also affect the solubility of a

molecule• Molecules with OH groups are able

to form hydrogen bonds with surrounding water molecules Soluble in water


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Example 21-4AExample 21-4A

Example 21-4BExample 21-4B


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Length of Carbon Length of Carbon ChainsChains• Molecules with higher molecular

masses have higher m.p., b.p. and density Higher molecular masses

Large molecular sizes Stronger London dispersion

forces among molecules


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Length of Carbon Length of Carbon ChainsChains• Molecules with branched chains

b.p. and density lower than its straight-chain isomer

Straight-chain isomers have greater surface area in contact with each other Greater attractive force among the molecules


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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21)Length of Carbon Length of Carbon

ChainsChains• Molecules with branched chains

m.p. higher than its straight-chain isomer

Branched-chain isomers are more spherical Packed more efficiently in solid state Extra energy is needed to break

down the efficient packing


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Example 21-4CExample 21-4C

Check Point 21-4Check Point 21-4


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Family General formula

Functional group

Example

Formula IUPAC name

Alkane RH (Nil) CH3CH3 Ethane

Alkene

RCH = CH2

RCH = CHRR2C = CHR

R2C = CR2

Carbon-carbon

double bond

CH2 = CH2 Ethene

AlkyneRC CHRC CR

– C C –Carbon-

carbon triple bond

HC CH Ethyne

Aromatic hydrocarbon ArH

Phenyl group

Benzene


R = CnH2n+1 –


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Functional group

Example

Formula IUPAC name

Haloalkane RX X

halo groupCH3Cl Chloromethane

Alcohol ROH OHhydroxyl group

CH3OH Methanol

Ether RO R O oxy group

CH3 O CH3

Methoxymethane

Aldehyde

carbonyl group

Methanal

R = CnH2n+1 –



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Functional group

Example

Formula IUPAC name

Ketone

carbonyl group

Propanone

Carboxylic acid

carboxyl group

Ethanoic acid

AmineRNH2

R2NHR3N amino group

CH3NH2 Methylamine

Nitrile RCN C Nnitrile group

CH3CN EthanenitrileR = CnH2n+1 –



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Functional group

Example

Formula IUPAC name

Ester

ester group

Methyl ethanoate

Acyl halideacyl halide

group

Ethanoyl chloride

Amide

amide group

Ethanamide

R = CnH2n+1 –



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Functional group

Example

Formula IUPAC name

Acid anhydride

acid anhydride group

Ethanoic anhydride

R = CnH2n+1 –



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The END


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(a)How was organic chemistry defined before 1800s? Answer

(a) The knowledge of organic and inorganic compounds was raised during the 1780s. Scientists defined organic chemistry as the study of compounds that could be obtained from living organisms. They believed that the synthesis of organic compounds took place in living organisms only.


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(b) How is organic chemistry defined nowadays?

Back

Answer(b) Nowadays, scientists have discovered that

many organic compounds can be synthesized from inorganic substances. The updated definition of organic chemistry is the study of carbon compounds, except for carbon monoxide, carbon dioxide, carbonates, hydrogencarbonates, carbides and cyanides. These compounds have been traditionally classified under inorganic chemistry.


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Why is carbon able to catenate?

Back

AnswerThe ability to catenate of carbon is chiefly due to the high strength of the CC single bond (bond enthalpy of C C single bond is 356 kJ mol-1).


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Would you expect silicon, which is just below carbon in the Periodic Table, to catenate to form diverse molecular structures? Explain your answer. Answer

Silicon, unlike carbon, does not catenate to form diverse molecular structures. Carbon is able to catenate because carbon atoms have a relatively small atomic size. This enables a carbon atom to form strong covalent bonds with other carbon atoms. However, due to the greater atomic size of silicon, its ability to catenate is much lower than that of carbon.

Back


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Would you expect sulphur, which has an electronegativity value very close to carbon, to catenate? Why?

Back

AnswerThe electronic configuration of sulphur is 1s22s22p63s23p4. It has only two unpaired electrons. Its atomic size is larger than that of carbon. So it has a much lower tendency to catenate than carbon.


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Identify the functional group(s) in the following compounds:(a)

Answer(a) Carbon-carbon double bond ( ) and

chloro group (Cl)


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Identify the functional group(s) in the following compounds:(b)

Answer(b) Carbonyl group ( )


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Identify the functional group(s) in the following compounds:(c)

Answer(c) Amino group ( ) and carboxyl group

( )

Back


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To which homologous series does each of the following compounds belong?(a)

Answer

(a) Ester


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To which homologous series does each of the following compounds belong?(b)

Answer

(b) Amide


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To which homologous series does each of the following compounds belong?(c)

Answer

(c) Acid anhydride

Back


3A71


State whether each of the following pairs of compounds belongs to the same homologous series. Explain your answer.(a)

Answer(a) No, the first one is a carboxylic acid and the

second one is an ester.


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State whether each of the following pairs of compounds belongs to the same homologous series. Explain your answer.(b)

Answer(b) Yes, both of them are alcohols.


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State whether each of the following pairs of compounds belongs to the same homologous series. Explain your answer.(c)

Answer(c) No, the first one is an amide and

the second one is an amine.

Back


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(a) Name the homologous series of organic compounds that contain oxygen atoms in their functional groups. Answer(a) Alcohol, ether, aldehyde, ketone,

carboxylic acid, ester, acyl halide, amide and acid anhydride


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(b) Identify and name the functional groups in glucose which has the following structure.

Answer

(b) OH (hydroxyl group) and O (oxy group)


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(c) Identify and name the functional groups in the following compounds:

Answer

Back

(c) Br (bromo),

(aldehyde),

(acyl chloride),

(carbon-carbon double bond) groups


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Why is oil immiscible with water?Answer

Oil molecules do not have free OH groups, so they cannot form hydrogen bonds with water molecules.

Back


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The relative molecular mass of glucose is 180.0, but it is soluble in water. Why?

AnswerGlucose molecules have OH groups, so they are able to form hydrogen bonds with water molecules. Therefore, glucose is soluble in water despite it has a high molecular mass. Back


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Despite the fact that butan-1-ol and ethoxyethane have the same relative molecular mass, they have very different boiling points. The boiling points of butan-1-ol and ethoxyethane are 117oC and 35oC respectively. Explain the difference. Answer


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There is an OH group in butan-1-ol. Thus, butan-1-ol molecules are able to form hydrogen bonds with one another and the energy required to separate butan-1-ol molecules would be much greater. Whereas for ethoxyethane, the attraction among the molecules is weak van der Waals’ forces only. The amount of energy required to break the forces would not be great. Therefore, the boiling point of ethoxyethane is lower than that of butan-1-ol.

Back


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Explain why propan-1-ol is soluble in water but

1-chloropropane is insoluble in water.

AnswerThe OH group of propan-1-ol molecules enables it to form hydrogen bonds with water molecules. Thus it is soluble in water. Although 1-chloropropane is a polar molecule, it does not form hydrogen bonds with water molecules. So it is insoluble in water.

Back


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Which molecule would have a higher boiling point, 1-bromobutane or 2-bromobutane? Why?

Answer1-bromobutane would have a higher boiling point. 1-bromobutane is a straight-chain molecule while 2-bromobutane is a branched-chain molecule. Straight-chain molecules have a greater surface area in contact with each other, so greater intermolecular forces exist among the molecules. Higher energy is required to break down the intermolecular forces among the molecules of 1-bromobutane.

Back


3A83


1-Chlorobutane and 2-chloro-2-methylpropane have the same molecular mass, yet their melting points differ. The melting point of 1-chlorobutane is –123oC while that of 2-chloro-2-methylpropane is –27.1oC. Explain the difference.

Answer


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Melting point is a measure of how efficient the molecules are packed together in the solid state instead of just comparing the van der Waals’ forces among molecules. Hence melting point is a function of the efficient packing of molecules but not the contact surface area. 1-Chlorobutane is a straight-chain molecule while 2-chloro-2-methylpropane is a branched-chain molecule. As 2-chloro-2-methylpropane is more spherical and symmetrical, its molecules are packed more efficiently in the solid state. 1-Chlorobutane is linear in shape and flattened, its packing in the solid state is not so efficient. Hence, it has a lower melting point than 2-chloro-2-methylpropane.

Back


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(a) What are the major factors that affect the physical properties of organic compounds?

Answer(a) The physical properties of organic compounds

are mainly affected by the structure of the functional groups, dipole moment of the molecule, the formation of hydrogen bonding between molecules, and the length of carbon chains of the molecule.


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(b) The melting point and boiling point of pentane are –130oC and 36.3oC respectively while the melting point and boiling point of 2,2-dimethylpropane are –15.9oC and 9.5oC respectively. Account for the difference in melting point and boiling point between the two isomers.

Answer


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(b) Pentane is a straight-chain molecule, while 2,2-dimethylpropane is a branched-chain molecule. Straight-chain molecules have a greater surface area in contact with each other than branched-chain molecules. Straight-chain molecules are held together by stronger intermolecular forces. Therefore, pentane has a higher boiling point than 2,2-dimethylpropane. Molecules of 2,2-dimethylpropane are more spherical in shape and are packed more efficiently in the solid state. Molecules of pentane are linear in shape and flattened, so their packing in the solid state is not efficient. Since extra energy is required to break down the efficient packing of 2,2-dimethylpropane, 2,2-dimethylpropane has a higher melting point than pentane.


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(c) Which molecule, hexane or cyclohexane, would have a higher melting point? Explain your answer.

Answer


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(c) Cyclohexane has a higher melting point than hexane. Molecules of cyclohexane are more spherical in shape and are packed more eff iciently in the solid state. Molecules of hexane are linear in shape and flattened, so their packing in the solid state is not efficient. Since extra energy is required to break down the efficient packing of cyclohexane, cyclohexane has a higher melting point than hexane.


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(d) Arrange the following molecules in increasing order of boiling points. Explain your answer.

Answer


3A91


(d) The boiling points increase in the order:butane < propanal < propan-1-olMolecules of butane are non-polar. Their molecules are held together by weak instantaneous dipole-induced dipole interactions. A relatively small amount of energy is required to separate the molecules in the process of boiling. Both propanal and propan-1-ol are polar molecules. Molecules of propanal are held together by relatively weak dipole-dipole interactions, while molecules of propan-1-ol are held together by intermolecular hydrogen bonds. Since the intermolecular forces present in molecules of propan-1-ol are stronger than those present in molecules of propanal, a larger amount of energy is required to separate the propan-1-ol molecules in the process of boiling.

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