New Way Chemistry for Hong Kong A-Level Book 3A1 Worksheet-01-Answers Introduction to Organic Chemistry Chem-160-Summer-2015

New Way Chemistry for Hong Kong A-Level Book 3A1

Worksheet-01-Answers

Introduction to Organic Chemistry

Chem-160-Summer-2015

New Way Chemistry for Hong Kong A-Level Book 3A2Dr Seemal Jelani2

21.1 What is Organic Chemistry (SB p.4)

(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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21.1 What is Organic Chemistry (SB p.4)

(b) How is organic chemistry defined nowadays?

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

Why is carbon able to catenate?

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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.

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

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Answer

The 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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21.3 Classification of Organic Compounds (SB p.14)

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

(a)

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

chloro group (Cl)

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(b)

Answer(b) Carbonyl group ( )

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(c)

Answer(c) Amino group ( ) and carboxyl group

( )

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

(a)

Answer

(a) Ester

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(b)

Answer

(b) Amide

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(c)

Answer

(c) Acid anhydride

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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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(b)

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

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(c)

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

the second one is an amine.

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

Why is oil immiscible with water?

AnswerOil molecules do not have free OH groups,

so they cannot form hydrogen bonds with

water molecules.

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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.

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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.

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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.

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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.

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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.

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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.

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(d) The boiling points increase in the order:

butane < propanal < propan-1-ol

Molecules 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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New Way Chemistry for Hong Kong A-Level Book 3A1 Worksheet-01-Answers Introduction to Organic Chemistry Chem-160-Summer-2015