Exam 1 - Free Response Questions

Question 1

Why do atoms bond?

Answer 1

Atoms bond to become stable valance electron configuration (8 valence electrons) by bonding with other atoms.

Question 2

What is electronegativity?

Answer 2

a measure of the tendency of an atom to attract a bonding pair of electrons

Question 3

What do you mean by dipole moment? Explain, considering water molecule as an example.

Answer 3

Dipole moment a measure of polarity of a molecule due to the difference of charge on either end of the molecule. Example> H2O has a permanent dipole moment due to H being much less electronegative than O creating a partial negative charge on the O’s and a partial + on H’s.

Question 4

Why is chloroform (CHCl3) polar whereas carbon tetrachloride (CCl4) is non-polar?

Answer 4

CHCl3 has dipole moment with vectors; CCl4 has a not net dipole moment

Question 5

NH3 is a polar molecule but BF3 is non-polar—justify.

Answer 5

Because F is evenly distributed around B but H is not evenly distributed around N

Question 6

Describe with suitable diagram(s), the hybridization of carbon in (i) methane

Answer 6

Methane (sp3)-
As seen in methane (CH4), carbon can form 4 bonds. The rationale behind this phenomenon is hybridization. Supporting evidence shows that 1s and 3 p orbitals are being combined to form hybrid orbitals, allowing polyatomic molecule to have 25% s character and 75% p character. Thus, we call methane a sp3-hybridized molecule.

Question 7

Describe with suitable diagram(s), the hybridization of carbon in ethylene

Answer 7

Ethylene(sp2)-
Sp2: Unlike methane, ethylene is shaped differently, despite the fact that the carbon in ethylene has the same electron configuration. What accounts for this difference? Supporting evidence shows that the carbon in an ethylene molecule is sp2 hybridized. This means that 1s orbital is being mixed with 2 p orbitals.

Question 8

Describe with suitable diagram(s), the hybridization of carbon in acetylene ( pic

Answer 8

Acetylene(sp)-
Sp: Supporting evidence shows that acetylene is an sp molecule. This means that 1s orbital is being mixed with 1 p orbital

Question 9

What do you mean by (i) σ-bond and (ii) π-bond? Explain.

Answer 9

sigma bonds are two orbitals overlaping head on and pi bonds are the lobes overlapping

Question 10

Explain the terms “resonance contributor” and “resonance hybrid” considering benzene as an example

Answer 10

RC: diff. structures of same molecule each representing a different form that a molecule can take
RH: a dipiction of a molecule showing an intermediate form of all the RC in one
model
Benzene can have 2 different configurations of 3 double bonds and
Benzenes res. hybrid would be drawn with 6 dotted lines representing possible double bonds.

Question 11

What makes resonance structure have decreased stability?

Answer 11

Electron delocalization, an increased amount of potential energy or resonance

Question 12

The double bond in cyclohexene can be oxidized by potassium permanganate (KMNO4), whereas the double bonds in
benzene do not undergo this oxidation reaction — Justify.

Answer 12

Benzene can not react with KMNO4 because of its resonance structure. Since it has this resonance structure its more stable and therefore does not react.

Question 13

Draw the orbital diagram of benzene.

Answer 13

Come back to this

Question 14

Benzene is aromatic whereas 1,3,5-hexatriene is nonaromatic — Explain.

Answer 14

1,3,5-hexatriene is not cyclic

Question 15

Cyclooctatetraene is a cyclic molecule with conjugated π-system having 8 π electrons but it is non-aromatic — Why?

Answer 15

is not polar

doesnt follow Huckel’s rule

Question 16

Write the two aspects of the electrophilic aromatic substitution reaction that are affected by a substituent? OR how the rate of the reaction and orientation are affected in EAS?

Answer 16

1.Presence of electron donating groups on the benzene makes the second eletrophilic substitution faster and the second substituent come and attack at either para or ortho position
2. Presence of electron withdrawing groups on the benzene makes the second electrophilic substitution less faster and the second substituent come and attack at meta position

Question 17

Define the terms: aromatic, non-aromatic and antiaromatic with examples of cyclopentadienyl cation, cyclopentadienyl anion and cyclopentadienyl radical.

Answer 17

the cyclopentadienyl anion is aromatic because it follows Huckel’s rule (4n+2) and it is also because it is planar, and it has two extra electrons which cause aromaticity.
Cyclopentadienyl radical has one extra electron so it does not follow Huckel’s or anti-aromatic rule (4n). The cation version will follow 4n rule; making it anti-aromatic.

Question 18

Write the four requirements of a molecule to be aromatic.

Answer 18

1.)Planar structure
2.)Conjugated double bonds
3.) follows (4n+2)
4.) 4 pi electrons

Question 19

Explain Hückel’s Rule of aromaticity with examples.

Answer 19

an aromatic compound must have (4n + 2) π electrons, where n = 0, 1, 2, 3….

example: cyclobutadiene meets most of the requirements to be aromatic, but it is NOT because it has 4 π electrons - it is anti aromatic. Benzene, on the other hand, with 6 π electrons, IS aromatic
Explain Hückel’s Rule of aromaticity with examples.

Question 20

Cyclobutadiene is not available to purchase directly from the market — Why?

Answer 20

It is unstable and it reacts very fast

Question 21

Clarify the terms: (a) Bonding MO (b) Antibonding MO (c) HOMO (d) LUMO

Answer 21

A) where 2 molecules, 2 p-orbitals of similar phase overlap
B) higher energy, becuase it destabilizing node results 2 p-orbitals of opposite phase overlap
C) highest occupied molecular orbital, highest energy orbital not containing electrons
D) lowest occupied molecular orbital, lowest energy orbital not containing electrons.

Question 22

Pyrrole is a heteroaromatic five membered (I) organic base (pKa = 0.4) whereas pyridine is a heteroaromatic six membered (II) organic base (pKa = 5.25). Explain why pyridine is 13 times more basic than pyrrole. (Pic question 20)

Answer 22

pyrrole is more acidic because the hydrogen is available, and it is not part of the ring system, and it is more susceptible to reach perride

Question 23

Explain with appropriate figures why o- and/or p- substitution are fovored for EDG whereas m-substitution is preferred for EWG in electrophilic aromatic substitution.

Answer 23

The double bond will not make the meta position the most stable because the (+) charge in carbon will go either in ortho and para position

Question 24

Halogens are electron withdrawing but are ortho/para director in electrophilic aromatic substitution-Explain

Answer 24

Halogens atoms are highly negative, but have unshared pairs of electrons that can be resonance donated to carbocation

Question 25

What is electrophilic acylium cation? How does it form and affect Friedel-Crafts acylation reaction?

Answer 25

resonance stabilized, + charged electrophile; formed by AlCI3 ionizing C-X bond of acid chloride; positive carbon is eletrophilic site to react in EAS

Question 26

Write the mechanism of the Friedel-Crafts acylation of anisole with acetic anhydride. ( pic question 24)

Question 27

Discuss the limitations in electrophilic aromatic substitutions with examples.

Answer 27

-molecule must be aromatic -acylation can be used only to give ketones. -F-C reactions dont occur with NH2 groups. -Benzene ring deactivated by EUG is not electron rich enough do undergo F-C reaction.

Question 28

Go over the examples of the four rules related to additivity of substituent effects in the case of EAS.

Answer 28

Ist rule: if directing effects of 2 subtituents reinforce each other the predicted product predominates 2nd rule: if directing effects of 2 subtituents oppose each other the more activating group dominates 3rd rule: substitutions almost never occur between 2 subtituents meta to each other 4th rule:with a bulky p-director and/or bulky electrohile, para subs predominates

Question 29

Ethyl benzene is treated with (i) Br2 and FeBr3 and (ii) Br2 and light or heat separately. Do you think the products will be same? Justify your answer.

Answer 29

Yes, because the difference # of atoms attracted to it would react differently and attack at different position

Question 30

Go through the oxidation and reduction products of substituted benzenes. Which reagents are used for (i) Clemmensen reduction and (ii) Wolff-Kishner reduction?

Answer 30

i Uses Zn and Hg in presence of strong acid ii.) uses (NH2NH2) and strong base (КОН)

Question 31

Two hydrogen atoms can combine to form H2 but two He atoms cannot form He2 — Justify from molecular orbital viewpoint.

Answer 31

Two He atoms came from He2 whereas two H atoms on H2 form because number of electrons of HOMO must be more than number of electrons in LUMO.

Question 32

What is ipso substitution? Give an example [structures need to be drawn].

Answer 32

Describes two substituents sharing the same ring position in an intermediate compound in an electrophilic aromatic substitution.

Question 33

Why aromatic compounds are more stable than their aliphatic counterpart? Draw the MO diagram of benzene.

Answer 33

Aromatic compounds are more stable than aliphatic counterparts because they have lower energy, more stability.

Question 34

What is the relation between numbers of node with energy of the orbital?

Answer 34

More the nodes that are the more the energy it has and the less stable it becomes

Question 35

Cycloheptatrienylium cation is more stable than Cycloheptatriene — Why?

Answer 35

Planar, follows Heckel's rules making it more stable, not planar and not aromatic

Question 36

Cyclopropenyl cation is a planar molecule. Is it "Aromatic", "Non-aromatic" or "Antiaromatic"?

Answer 36

Aromatic

Question 37

Imidazole with 8 π electrons is aromatic — Why?

Answer 37

Because it only uses one lone pair in the conjunction Pi system

Question 38

Furan with 8 π electrons is aromatic — Why?

Answer 38

Because one of the lone pair of electrons of the oxygen atom is delocalized in the ring

Question 39

Thiophene with 8 π electrons is aromatic — Why?

Answer 39

Because one of the lone pair of electrons of the sulfur atom is delocalized in the ring

Question 40

Pyrene with 16 π electrons is aromatic — Justify.

Answer 40

Becuase the Pi bond in the middle of the ring is not participating in resonance

Question 41

The compound [10] Annulene is not aromatic — Justify.

Answer 41

Hydrogens interfere preventing polarity

Question 42

What do you mean by "Pericondensed" and "Catacondensed" polycyclic aromatic hydrocarbons (PAHs)? Give two examples in each case (structures are required).

Answer 42

Pericondensed: more than one internal carbon node, Pyrene and coronene Catacondensed: No internal carbon vertices Naphthalene and phenanthrene

Question 43

EAS can be considered as Lewis acid-Lewis base reaction — Justify.

Answer 43

Because it receives an electrophile attacking a carbon atom

Question 44

The boiling points of o-, m-, and p-Xylenes are 144 °C, 139 °C and 138 °C respectively — Justify.

Answer 44

boiling point dependent upon dipole moment; ortho>meta›para for disubstituted benzenes

Question 45

How the electrophilicity of bromine (a weak electrophile) can be increased by using FeBr3

Answer 45

Br is weak electrophile; when FeBr3 used, Br electrophilicity increased bc creates polarized bromine; polarized bromine serves as Br+ for electrophile to be added & form carbocation

Question 46

Write the name of the electrophiles in nitration and sulfonation reactions?

Answer 46

Nitration- Nitronium ion (+NO2) Sulfonation- Sulfonic ion (+SO3H)

Question 47

Write the difference between 'reagent' and 'catalyst' of a reaction.

Answer 47

Catalyst accelerates the rate of the reaction but it doesn't get by the reaction, and the reagent initiates the reaction and it gets consumed

Question 48

Write the name and resonance structures of the electrophile in Friedel-Crafts acylation reaction.

Answer 48

Acylium ion

Question 49

Why n-Propylbenzene cannot be synthesized by Friedel-Crafts alkylation reaction?

Answer 49

because alkylation will give a product of isopropylbenzene it will be acylation that will yield propylbenzene by reduction