Sunday Week 1 Solutions PDF

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Week$1$Problem$Set$(Solutions)$ 2/5,%2/6,%2/7% % Concepts%Covered% Drawing%structures%from%names% Hybridization% Resonance% Constitutional%isomers%

Krishna(Mallem(Week(1(( ( 1. Draw(the(following(molecules.(( ( a)! 2,3(–(dimethylcyclopentan(–(1(A(ol(( ( ( OH ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( a)! 4(–(isopropyl(–(1(–(vinylcyclohex(–(1(A(ene((( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (

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Krishna(Mallem(Week(1(( ( 4. Label(the(hybridization(of(each(heteroatom(and(carbon(atom.(Then,(draw(all(the(contributing( resonance(structures(with(the(most(number(of(complete(octets.(Make(sure(to(include(all(formal( charges.(( ((

N

N

O

O B

B

N

O B

H 2N N

O B

H 2N

O B

( (

O B

Week 1 - David Armanious

Hybridization, Resonance, Constitutional Isomers

5. Hybridization and Resonance Label the hybridization of all heteroatoms, cations, and anions. Then, draw all CRS where all atoms have a full octet and circle the best resonance structure(s). Note: there may not be a resonance structure.

a. H N

N H

sp 2

H N

N H

sp 3

The first resonance structure is the best because all atoms are neutral—there is no separation of charge. b. sp 2 O

sp 2

O

B

O

B

B

O

sp 2

O

O

sp 2

The second resonance structure is the best because all atoms have a full octet and because it has the least separation of charge. c. O

sp 2

O

O

O

sp 2

These two circled resonance structures are the best because oxygen is more electronegative then carbon and is more stable than carbon with a negative charge.

O

Week 1 - David Armanious

Hybridization, Resonance, Constitutional Isomers

d. sp 2

sp 2

O

NH 2

O

NH 2 O

NH 2

O

O

O

sp 2 The second resonance structure is the best because nitrogen is better at stabilizing a positive charge than oxygen because of its lower electronegativity. e. sp 3 O-

sp N

This molecule has no resonance structures. The intervening carbon is sp3 hybridized, and so it has no electrons in a π orbital to participate in resonance. The oxygen, then, because it cannot participate in any resonance structures, is also sp3 hybridized.

f. For this problem, identifying the best resonance structure is an optional challenge. -N

sp

N+

NH

sp

sp 2

This is a tricky problem because the leftmost nitrogen at first appears to be sp2 hybridized. In one of the major CRS, however, that nitrogen is sp hybridized. Like in other cases, an atom will “rehybridize” in order to participate in that resonance structure.

N

N

NH

N

N

NH

For determining the best resonance structure, the separation of charges rule dominates. Because the N=N bond length is shorter than the N-N bond length, the positive and negative charges in the second resonance structure are closer to each other, and therefore more stable.

Week 1 - David Armanious

Hybridization, Resonance, Constitutional Isomers

6. Constitutional Isomers First, calculate the degrees of unsaturation for the given chemical formula. Then, draw all constitutional isomers. (Don’t draw stereoisomers such as cis/trans isomers.)

!. !. =

!"# !"#$%&' ∗ 2 + 2 − !"# !"#$%&'() 2

Add 1 to Num Hydrogens for each halogen (F, Cl, Br, I) in the chemical formula. Ignore sulfur and oxygen. Subtract 1 from Num Hydrogens for each nitrogen in the chemical formula. The trend can best be seen by thinking of adding each of those atoms to methane: Formula CH4 CH3X (X = F, Cl, Br, I) CH3OH CH3NH2 # Hydrogens 4 3 4 5

a. C3H7N. Do not draw any cyclic isomers. 3 ∗ 2 + 2 − (7 − 1) =1 2 Because we can’t draw cyclic isomers, the unsaturation must come from a double bond. !. !. =

NH 2 NH 2

NH 2

NH NH

H N

N

N

Week 1 - David Armanious

Hybridization, Resonance, Constitutional Isomers

b. C6H12. Each isomer must be cyclic. !. !. =

6 ∗ 2 + 2 − 12 =1 2

Because each isomer must be cyclic, that must be the only form of unsaturation for each structure. Always try to use a systematic approach when drawing constitutional isomers.

Krishna(Mallem(Week(1(( ( 7. Draw(all(constitutional(isomers(of(C4H8O(that(do(not(contain(any(sp2(hybridized(orbitals.(( ( ( ( O ( ( ( ( ( ( OH ( ( O O ( ( ( ( ( ( ( OH ( ( O ( ( ( O ( ( ( ( ( ( ( ( O ( OH ( ( ( ( ( ( ( ( ( ( (

( (

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