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Alcohols Flashcards

1
Q

What is Beta elimination?

A

A reaction in which a molecule, such as HCl, HBr, HI or HOH is split out or eliminated from adjacent carbons.

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2
Q

What is a fundamental difference between Sn1 and Sn2 reactions?

A

The timing of the bond breaking between carbon and the leaving group and of bond forming between carbon and the nucleophile.

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3
Q

What are two ways the backside attack by the nucleophile is facilitated in Sn2 reactions??

A
  1. Carbon atom is a partial positive charge and attracts electron-rich nucleophile
  2. Electron density of nucleophile assists in breaking C-Lv bond
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4
Q

Why is a backside attack the most energy efficient reaction geometry for an Sn2 reaction?

A

The antibonding orbital of the C-Lv bond is concentrated on the backside so the electron density of the nucleophile can populate it most easily from the backside.

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5
Q

Why does the transition state have a higher internal energy than the reactant or product in an Sn2 reaction?

A

There is distortion from optimal bonding arrangements [the transition state is trigonal pyramidal with one bond breaking and one bond forming]

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6
Q

What is a solvolysis reaction?

A

A nucleophilic substituion in which the solvent is also the nucleophile.

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7
Q

Why is the ionization of the C-Lv to form a carbocation intermediate slower than the breaking of the C-Lv bond in an Sn2 reaction?

A

There is no nucleophile helping to add electron density to the antibonding orbital of the C-Lv bond.

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8
Q

Why do Sn1 reactions sometimes result in only partial racemization of the product?

A

Backside attacks (inversion of configuration) happen more easily because after the Lv and carbocation dissociate, they remain as an ion pair, so the Lv- partially blocks the nucleophile from the front side.

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9
Q

The rate of Sn1 reactions are governed by mainly what factor?

A

The stability of the carbocation intermediate.

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10
Q

The rate of Sn2 reactions are governed by mainly what factors?

A

Steric factors, and transition states are particularly sensitive to bulky groups at the site of reactions.

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11
Q

Rank the stability of carbocations from least to most stable

A

methyl < primary alkyl < (secondary alkyl, primary allylic, primary benzylic) < (tertiary alkyl, secondary allylic, secondary benzylic) < (tertiary allylic and tertiary benzylic)

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12
Q

What is the significance of the carbon hybridization in the formation of the carbocation and Sn1 reactions?

A

Sn1 reactions rarely occur on Sp2 and never on Sp carbons because of the instability of the carbocation.

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13
Q

What can you infer about the reaction mechanism from the degree of substitution of the alkyl chain?

A

Primary are always Sn2, Tertiary are always Sn1, and Secondary depend on nucleophile and solvent.

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14
Q

What is the significance of beta branching in Sn2? (degree of substitution of carbon adjacent to carbon holding leaving group)

A

As beta branching increases, relative rate of Sn2 decreases. For all practical purposes, three beta-branches do not undergo Sn2 reactions.

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15
Q

What is the significance of carbon hybridization in Sn2 reactions?

A

Sn2 reactions never occur on sp2 or sp hybridized carbons.

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16
Q

The best leaving groups have strong conjugate _______

A

Acids; this means the anion is stable.

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17
Q

What is a protic solvent?

A

A solvent that has hydrogen bonds

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18
Q

What is a polar solvent?

A

A solvent with a dielectric constant greater than 15; can insulate opposite charges.

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19
Q

What are three (3) common polar protic solvents?

A
  1. Water
  2. Low molecular-weight alcohol (methanol, ethanol)
  3. Low molecular-weight carboxylic acids (formic acid)
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20
Q

What are four (4) common polar aprotic solvents?

A
  1. Dimethyl Sulfoxide (DMSO)
  2. Acetonitrile
  3. Acetone
  4. DMF
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21
Q

What are three (3) common nonpolar aprotic solvents?

A
  1. Diethyl ether
  2. Toluene
  3. Hexane
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22
Q

What are two (2) common moderately polar aprotic solvents?

A
  1. Dichloromethane

2. tetrahydrofuran (THF)

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23
Q

What is a common moderately polar protic solvent?

A

Acetic acid

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24
Q

What solvent best facilitates an Sn1 reaction and why? (2 components to why)

A

Polar protic because they are able to (1) keep opposite charges separated and (2) stabilize positive and negative species

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25
How does a polar protic solvent affect the reaction coordinate diagram for an Sn1?
Polar protic solvents decrease the internal energy of the charged species (the carbocation intermediate) relatively more than they do the neutral reactants, so the activation energy decreases.
26
What solvent best facilitates an Sn2 reaction and why?
Polar aprotic solvents because hydrogen bonds do not create a solvation shell around the nucleophile (and make it require more energy to reach transition state)
27
Polar protic solvents favor what type of reaction?
Sn1, E1 and E2
28
Polar aprotic solvents favor what type of reaction?
Sn2
29
Nonpolar solvents favor what type of reaction?
Sn2 and E2
30
What is nucleophilicity?
A kinetic property related to the rate a nucleophile causes nucleophilic substitution on a reference compound.
31
What is a general relationship between basicity and nucleophilicity?
Unhindered strong bases are good nucleophiles.
32
Describe the nucleophilicity of carboxylate anion (conjugate base of carboxylic acid)
Moderate nucleophiles
33
What can be said about the relationship between the interaction of the nucleophile and the solvent and its nucleophilicity?
All other factors being equal, the stronger the interaction of the nucleophile with the solvent, the lower its nucleophilicity.
34
Polar aprotic solvents solvate what type of ionic species better?
Cations, meaning the anions are freer and less encumbered.
35
In polar aprotic solvents, what is the relationship between nucleophilicity and basicity?
Nucleophilicity parallels basicity (F- > Cl- > Br- > I-)
36
In polar protic solvents, what is the relationship between nucleophilicity and basicity?
Nucleophilicity is opposite of basiciy (I- > Br- > Cl- > F-)
37
What are two (2) common examples of nucleophiles that are strong because they have no steric hindrance?
1. Azide | 2. Cyanide
38
What is the relationship between steric hindrance and nucleophilicity?
The more sterically hindered the nucleophilic atom is, the less nucleophilic it is.
39
What are three (3) common bases used for E2 reactions?
1. Hydroxide 2. Alkoxide 3. Amide anions
40
At a secondary carbon, which 2nd order reaction type predominates in the presence of an alkoxide?
E2
41
What is significant about a sterically hindered base when choosing the product of a reaction?
They are not good nucleophiles so they will only participate in elimination; They will also remove the less sterically hindered hydrogen (non-Zaitsev)
42
How do you predict the stereochemistry of an E2 product?
There must be anticoplanar elimination.
43
What are three possible carbocation reactions in a 1 type reaction?
1. Removal of beta hydrogen to form alkene (E1) 2. reaction with nucleophile (sn2) 3. rearrangement followed by (1) or (2)
44
How do you predict the ratio of Sn1 to E1 for first order reactions?
1. Heat favors elimination | 2. Weak base favors substitution
45
What are the two guiding principles when comparing the ratio of Sn2 to E2 products?
1. increased alpha-carbon or beta-carbon branching favors E2 because it (1) increases steric hindrance, retarding Sn2 and (2) increases stability of alkene. 2. increased nucleophilicity favors Sn2; increased basicity favors E2
46
Why does increased temperature favor E2 reactions over Sn2?
E2 reactions create three molecules (an alkene, conjugate acid, and free leaving group), increasing entropy. At higher temperatures, the entropy term in the Gibbs free energy equation becomes more significant, so the competing reaction with the higher entropy dominates.
47
Strong bases almost always favor what reaction?
E2
48
When do strong bases not favor E2?
When R-group is primary and sterically unhindered; in this case the strong base must be bulky for E2 to be favored, otherwise the base will act as a nucleophile and undergo Sn2
49
What is an example of a strong nucleophile but a weak base?
Iodide, cyanide, Azide
50
What is an example of a strong base but a weak nucleophile?
Tert-butoxide
51
When comparing the competition of Sn1, Sn2, E1, E2, what order do you consider the factors?
1. Degree of Substitution of R-group 2. Is nuc/base a strong base? 3. Is it a good nucleophile? (Consider effect of solvent) 4. If it is not a good nucleophile, it may be a first order reaction.
52
Good nucleophiles are generally what?
Anions
53
Moderate nucleophiles are generally what?
Neutral but partially negative or containing one or more lone pairs
54
Poor nucleophiles are generally what?
Polar protic solvents
55
How does EM radiation raise the energy level of a molecule?
The frequency of the propagating wave matches the vibrational mode of the molecule, and constructive interference of the +/- aspect of the EM wave couples with the dipole moment of the vibrating molecule and increases the energy.
56
What are two (2) wave numbers for Sp3 C-H bonds stretching? [IR spectroscopy]
1. 2850 cm-1 | 2. 2930 cm-1
57
What is the wave number for sp3 C-H bond scissoring? [IR spectroscopy]
1470 cm-1
58
What is the wave number for sp2 C-H bond stretching? [IR spectroscopy]
3050-3150 cm-1
59
What is the wave number for alkene C=C bond stretching? [IR spectroscopy]
1645-1670 cm-1
60
If you suspect an alkene based on C=C bond, what other wave numbers should you check for? [IR spectroscopy]
1. 890 cm-1 2. 910 cm-1 3. 970 cm-1 4. 990 cm-1
61
What is the wave number for the alkyne C=-C bond stretching? [IR spectroscopy]
2200 cm-1
62
If you suspect an alkyne based on C=-C bond, what other wave numbers should you check for? [IR spectroscopy]
1. 3300 cm-1 | 2. 600-700 cm-1
63
What is the wave number indicative of an alcohol O-H bond? [IR spectroscopy]
3200-3600 cm-1
64
What wave number indicates the C-O bond of a primary alcohol? [IR spectroscopy]
1050 cm-1
65
What wave number indicates the C-O bond of a secondary alcohol? [IR spectroscopy]
1150 cm-1
66
What wave number is most indicative of an aldehyde? [IR spectroscopy]
2720 cm-1
67
What wave number range indicates a ketone?
1715-1725 cm-1
68
What is the wave number range for an alkyl halide?
500-700 cm-1
69
What are four (4) methods of preparing alcohols?
1. Hydration of alkenes 2. Grignard reactions 3. Reduction of carbonyl oxygen with metal hydrides (LiAlH4) 4. Catalytic hydrogenation
70
What are two sources of halide atoms for alcohol substitution reactions?
1. Using HCl or HBr | 2. Using an inorganic halide
71
What type of reaction (Sn1 or Sn2) do inorganic halide sources react with?
Sn2 always
72
What are four (4) examples of inorganic halides used for alcohol substitution
1. PCl3 2. PBr3 3. P, I2 4. SOCl2
73
What happens when an alcohol reacts with Tosyl Chloride?
The -OH of the alcohol reacts with the H of the TsCl and forms water; the remaining pieces form a Tosylate. (THIS IS A SPECIAL FORM OF THE ESTER FORMATION)
74
What is the benefit of forming a Tosylate from an alcohol?
It is an excellent leaving group and -OH is a terrible leaving group.
75
How can you form an ester from an alcohol?
The -OH of an acid (e.g. carboxylic acid) will form water with the -H of the alcohol and the remaining pieces will form an ester. (dehydration synthesis). Forming a tosylate is an example of this.
76
What are three (3) possible products when oxidizing an alcohol?
1. Aldehyde 2. Ketone 3. Carboxylic acid
77
What are secondary alcohols oxidized to?
Ketones
78
What are primary alcohols oxidized to?
Aldehyde or Carboxylic acid
79
What are three (3) oxidizing agents used for alcohols?
1. Na2Cr2O7/H2SO4, H20 2. KMNO4 3. Pyridinium chlorochromate (PCC)
80
What is significant about using PCC to oxidize a primary alcohol?
It stops after the first step (forming an aldehyde)
81
What happens if you use an oxidizing agent with a tertiary alcohol?
No reaction. Tertiary alcohols are not easily oxidized.
82
How do you form the common name of an alkyl halide?
Name of alkyl group + Name of halide
83
Compare boiling temperatures of alkyl halides to alkanes with similar number of carbons
Alkyl halides have higher boiling point
84
Compare boiling points of alkyl halides with same number of carbons
The bigger the halogen the higher the boiling point
85
Compare the boiling point of alkyl halides with alkanes of similar molecular weight
Cl > Alkanes | Alkanes > Br & I [the heavier atoms reduce number of carbons, concentrate M.W. and reduce surface area]
86
What are four (4) preparation methods of alkyl halides
1. Prepare from alcohols 2. Halogenate alkane 3. Electrophilic addition to alkene 4. Halide exchange
87
Compare the density of alkyl halides with water
R-Cl > H2O | H2O > R-Br, R-I
88
How are simple ethers named?
Name the alkyl groups attached to oxygen (order them alphabetically)
89
How are complex ethers named if there is a single chain?
Name the part attached to the oxygen that is not a part of the main chain as an alkoxy derivative
90
How do you name epoxides? (Two ways)
1. Name the two carbons the oxygen is bonded to and then name the chain as a single alkane (e.g. a three carbon molecule with oxygen tethered to the first and second is 1,2-epoxypropane 2. Call the two carbons the oxygen is tethered to "oxirane" and name the alkyl group attached to it (a single methyl group attached to the second carbon of oxirane is 2-methyloxirane)
91
Describe the solubility of ethers and epoxides
Very small dipole moment: 2-3 carbons are miscible 4 carbons 7% solution 5 carbon 1% solution
92
Describe the boiling points of ethers and epoxides
Similar boiling points to hydrocarbons of about the same size
93
What effect does the position of the oxygen in the ether/epoxide have on the boiling point?
The closer to the end of the molecule the oxygen is, the higher the boiling point.
94
What decomposition occurs if an ether/epoxide is left standing?
Accumulate peroxides
95
What are three (3) methods for preparing ethers?
1. Symmetrical ethers prepared by dehydration of alcohols 2. Williamson Synthesis 3. Alkoxymercuration-demercuration
96
What are two (2) methods for preparing epoxides?
1. Internal Williamson synthesis | 2. Oxidation of alkenes
97
What reaction is used to produce a symmetrical ether?
2ROH @ 140 degrees celsius with H2SO4; dehydration synthesis.
98
What are the reagents for a williamson synthesis to prepare an ether?
Alkyl halide (R-X) and Alkoxide (Na+OR-)
99
What type of reaction is Williamson synthesis (E1, E2, Sn1, SN2)
Sn2
100
When planning a synthesis with a Williamson synthesis, how do you know where to break the bond?
The reagents that you create by "breaking the bond" must be favorable for an Sn2 reaction.
101
How do you create an alkoxide?
R-OH + Na -> H2 + NaOR-
102
How can you create an epoxide with an internal williamson synthesis on a halohydrin?
A strong base like NaOH will deprotonate the -OH in the halohydrin and the O- will perform an Sn2 reaction with the adjacent carbon containing the leaving group (e.g. Cl)
103
How can you create an ether with alkoxymercuration?
Oxymercuration uses the mercurinium ion in a water solvent so it adds an -OH. Alkoxymercuration uses an alcohol solvent so O-R is added instead. Remove the mercurinium ion by reducing with NaBH4.
104
How can you make an epoxide from an alkene?
Peroxyacids oxidize alkenes into epoxides.
105
What is an important reaction of ethers?
Cleavage by concentrated acids (HI or HBr)
106
Describe the reaction of HI/HBr cleaving an ether
the Oxygen of the ether is protonated and leaves with part of the ether as an alcohol (may react further to form water and alkyl halide); the other half is substituted by a halide.
107
What are two (2) important reactions that epoxides undergo?
1. Ring opening and backside attack | 2. Cleavage by bases
108
Describe the reaction of ring opening and backside attack by acids for epoxides
Oxygen is protonated by acid and opens up (forming carbocation that can best handle the positive charge) and then carbocation is back side attacked by nucleophile (e.g. halide)

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