2. Structure and Stability

Question 1

A molecule is said to be saturated if it has no what?

Answer 1

double bonds (pi bonds) OR rings.

Question 2

what is the formula for the degree of unsaturation? Explain the variables in the equation

Answer 2

DoUS = ((2n+2) - x) / 2

n = # of carbons 
x = number of hydrogens or any monovalent atoms (e.g. halogens)

Question 3

how do we treat oxygen and nitrogen when calculating degree of unsaturation?

Answer 3

oxygen is ignored
for any nitrogen containing compound, we replace N with 1C and 1H for the formula

((2n+2) -x) / 2

Question 4

what is a vinyl group?

Answer 4

an ethene functional group (1 carbon double bond)

Question 5

what is hybridization?

Answer 5

hybridization theory explains the pairs of electrons must occupy molecular orbitals (s,p,d,and f)

Question 6

Explain the stability/reactivity pattern of carbocations and carbanions.

Answer 6

stable unstable / reactive
carbocations 3 2 1 methyl

carbanions methyl 1 2 3

i.e.
primary/methyl carbocations are very reactive
tertiary carbanions are very reactive

Question 7

what two effects contribute to reaction intermediate stability?

Answer 7

inductive effects

resonance effects

Question 8

how can you explain why tertiary carbocation is the most stable carbonation?

Answer 8

because of inductive effects. the three alkyl groups surrounding the positive carbon are electron donators which reduces the positive charge on the central carbon.

Question 9

t or f, all resonance structures contribute equally.

Answer 9

false,

this can be true, however, in some cases one structure can contribute less or more than others.

Question 10

what is the hybridization of nitrogen in aniline? (aromatic amine). Why is it like this?

Answer 10

aniline has 4 resonance structures, some of which give the amine group a double bond. However, breaking the pi system in the benzene ring is unfavorable, and therefore some structures are worse than others in the resonance system.

therefore, the nitrogen is not technically sp3 or sp2 hybridized but rather a mix between sp3 and sp2.

Question 11

IF a molecule is in resonance, always consider how the hybridization is effected!!

Answer 11

true

Question 12

What three factors, in order, determine resonance structure stability (and therefore level of contribution)?

Answer 12

1. satisfy the octet rule (8 valence electrons)
2. minimize separation of charge
3. formal charge: negative charges on EN atoms, positive charges on less EN atoms.

Question 13

Explain the hybridization of the ether oxygen within an ester molecule.

Answer 13

The ether oxygen has two bonds and two lone pairs of electrons (4 electron groups). However, it is sp2 hybridized because one of its electron pairs exists in resonance with the double bonded oxygen of the ester molecule.

ALWAYS account for resonance.

Question 14

Can the lone pair of electrons on the NH2 group of aniline act as a nucleophile?

Answer 14

NO! This is because the lone electrons are participating in resonance.

Question 15

Why are carboxylic acids more acidic than alcohols?

Answer 15

When alcohols lose their proton, the negative charge is held on an oxygen (EN) which is good, making them more acidic than alkanes. However, carboxylic acids hold their negative charge on an oxygen that is also in resonance. This stabilizes the negative charge more making it more acidic.

Question 16

Why is carboxylic acid more acidic than phenol (OH with conjugated ring system)

Answer 16

both resonance structures for CA have the negative charge on oxygen. In phenol, 3/4 resonance structures have the negative charge on a carbon.

Question 17

What is the general order of acidity of organic compounds in order (n=9)?

Answer 17

Know strong acids (HX) > sulfonic acids (S=O) > carboxylic acids (C=O) > phenols > alcohols > ketones / aldehydes > sp > sp2 > sp3

Question 18

What is more acidic, and alkane or alkyne?

Answer 18

sp > sp2 > sp3

thus, alkynes are most acidic.

Question 19

What are inductive effects?

Answer 19

Nearby EN atoms pull electron density towards themselves which makes protons more delta positive. they also stabilize a negative charge.
(thus increasing a compounds acidity).

Question 20

t or f, the strength of inductive effects decreases with distance?

Answer 20

true, a compound with EN atoms closer to the alpha proton will be more acidic.

Question 21

How do substituents effect acidity of phenol’s (any compound in general)?

Answer 21

electron with-drawing groups will increase acidity (NO2), electron-donating groups with decrease acidity (NH2) .

Question 22

Why is NH2 considered an electron-donating compound?

Answer 22

Despite N being highly EN, it holds a lone pair of electrons that like to attack delta positive centers. Thus its electron-donating.

Question 23

What is a nucleophile?

Answer 23

A nucleophile is any compound with a lone pair of electrons OR a pi bond. Nucleophiles tend to have a negative charge or a delta negative partial charge.

Question 24

t or f, the strongest nucleophiles are compounds that poorly stabilize their negative charge.

Answer 24

true! nucleophiles are essentially Lewis bases. The worse they stabilize their charge, the more reactive (i.e. stronger) they are.

Question 25

Explain how you determine a nucleophiles strength (periodic trends) (n=3).

Answer 25

1. the larger the negative charger the stronger
2. Nu strength increases down a group (I > Br > Cl > F)
3. Nu strength increase to the left of a period (N > O)
   - since EN decreases to the left, atoms cannot bear the negative charge and thus make stronger Nu’s.

Question 26

Explain the leaving group ability of halogens.

Answer 26

The larger halogens can stabilize a negative charge better, and therefore iodine is the best leaving group.

LG I > Br > Cl > F

(and therefore HI is the strongest acid since it can dissociate the best )

Question 27

t or f, strong bases are good leaving groups.

Answer 27

FALSE, weak bases are good leaving groups. Strong bases are reactive on their own (which means they are not stable)

Question 28

What are many organic reactions acid-catalyzed? (hint: to do with the leaving group).

Answer 28

Because of LG stability. For example, OH- is a bad leaving group (strong base). IF we treat the solution with H+, the OH- becomes protonated. H2O is a good leaving group and thus the reaction will continue.

Question 29

What is ring strain?

Answer 29

certain cycloalkanes are not in ideal bond angles (cyclopropane and cyclobutane). Since they are unstable, they are much more reactive and are capable of undergoing hydrogenation reactions.

Cyclohexane and cyclopentane do not undergo these reactions because they experience much less strain (more stable)