Organic Chemistry Flashcards
(114 cards)
1
Q
List the steps of IUPAC nomenclature
A
- Find the longest carbon chain in the compound with the highest-order functional group; 2. Number the chain; 3. Name the substituents; 4. Assign a number to each substituent; 5. Complete the name
2
Q
In a molecule with two double bonds adjacent to each other and an alcohol, which functional group would take precedence in naming?
A
The alcohol would take precedence because the carbon to which it is attached has a higher oxidation state
3
Q
What are the common names for 2-propanol and ethanol?
A
2-propanol: isopropyl alcohol
ethanol: ethyl alcohol
4
Q
What is the difference between an aldehyde and a ketone?
A
An aldehyde has a carbonyl group at the end of the chain. A ketone has a carbonyl group somewhere in the middle of the carbon chain. Another way to think of this is that the carbonyl carbon of an aldehyde has at least one bond to a hydrogen atom, whereas the carbonyl carbon of a ketone is always bonded to two other carbons
5
Q
What suffixes are used for aldehydes and ketones; how are carbonyl groups names as a substituent?
A
Aldehydes are referred to with the suffix -al, while ketones are given the suffix -one. Carbonyl groups of both aldehydes and ketones are labeled as oxo- substituents (ketones may also be called ketone- substituents)
6
Q
For a molecule with a double bond, an aldehyde, and an alcohol, which functional group would determine the suffix when naming?
A
Ketones and aldehydes both take precedence over both alcohols and hydrocarbon chains, and the functional group that is the highest priority determines the suffix. Because the aldehyde is chain-terminating and therefore on carbon number 1, the aldehyde would determine the suffix when naming this compound
7
Q
What would be the names of the ester, amide, and anhydride derivatives of pentatonic acid? Assume the R group on the ester is -CH3 and that the amide is unsubstituted.
A
The ester derivative would be methyl pentanoate. The amide would be pentanamide. The anhydride would be pentanoic anhydride.
8
Q
What are physical properties?
A
characteristics of processes that don’t change the composition of matter, such as melting point, boiling point, solubility, odor, color, and density
9
Q
What are chemical properties?
A
have to do with the reactivity of the molecule with other molecules and result in changes in chemical composition; they are generally dictated by the functional groups in the molecule
10
Q
What property or properties do structural (constitutional) isomers have in common?
A
structural isomers share a molecular formula, and not necessarily anything else
11
Q
What is the difference between a conformational and a configurational isomer?
A
Conformational isomers are stereoisomers with the same molecular connectivity at different points of rotation around a single bond. Configurational isomers are stereoisomers with differing molecular connectivity
12
Q
What is a meso compound?
A
A meso compound contains chiral centers but also has an internal plane of symmetry. This means that the molecule is overall chiral and will not rotate plane-polarized light
13
Q
How is priority assigned under the Cahn-Ingold-Prelog priority rules?
A
Priority is assigned by atomic number: the atom connected to the stereocenter or double-bonded carbon with he highest atomic number gets highest priority. If there is a tie, one works outward from the stereocenter or double-bonded carbon until the tie is broken.
14
Q
Is the stereochemistry retained or inverted by switching a pair of substituents?
A
inverts stereochemistry
15
Q
Is the stereochemistry retained or inverted by switching two pairs of substituents?
A
retains stereochemistry
16
Q
Is the stereochemistry retained or inverted by rotating the molecule 90°?
A
inverts stereochemistry
17
Q
Is the stereochemistry retained or inverted by rotating the molecule 180°?
A
retains stereochemistry
18
Q
Summarize the quantum numbers
A
n: name - principal QN, describes - size, organization level - shell, possible values - 1 to infinity
l: name - azimuthal QN, describes - shape, organization level - subshell, possible values - 0 to n-1
ml: name - magnetic QN, describes - orientation, organization level - orbital, possible values - -l to +l
ms: name - spin QN, describes - spin, possible values - -1/2 or +1/2
19
Q
Which is more stable: a bonding orbital or an antibonding orbital? Which has higher energy?
A
Bonding orbitals are more stable than antibonding orbitals. Therefore, antibonding orbitals have higher energy than bonding orbitals.
20
Q
What differences would be observed in a molecule containing a double bond compared to the same molecule containing only single bonds?
A
The differences would be in bond length (shorter in double bond than single), bond energy (higher in double bond than single), and molecular rigidity (higher in double bond than single).
21
Q
Rank the following orbitals in decreasing order of strength: sigma bond, pi bond, double bond, triple bond
A
triple bond > double bond > sigma bond > pi bond. Remember that while an individual pi bond is weaker than a sigma bond, bond strength is additive. Therefore, double bonds are stronger than single, and triple bonds are stronger still.
22
Q
What is the s character of sp-, sp^2-, and sp^3-hybridized orbitals?
A
sp orbitals have 50% s character and 50% p character
sp^2 orbitals have 33% s character and 67% p character
sp^3 orbitals have 25% s character and 75% p character
23
Q
What are resonance structures? How does the true electron density of a compound relate to its resonance structures?
A
Resonance structures differ in their placement of electrons in hybridized p-orbitals and require bond conjugation to delocalize electrons in a molecule. The true electron density is a weighted average of the resonance structures of a given compound, favoring the most stable structures.
24
Q
When will an acid-base reaction proceed, based on the strength of the reactants and products?
A
An acid-base reaction will proceed when the acid and base react to form conjugate products that are weaker than the reactants
25
What does it mean for a molecule to be amphoteric? What biologically relevant molecules are also characteristically amphoteric?
Amphoteric species can act as either an acid or a base. Water, bicarbonate, and dihydrogen phosphate are common amphoteric species in biological systems.
26
How is pKa defined, and what does a low pKa indicate?
pKa = -log Ka, where Ka is the equilibrium constant for the dissociation of an acid. pKa indicates acid strength: a stronger acid has a lower (or even negative) pKa
27
What are some functional groups that classically act as acids? Bases?
Alcohols, aldehydes and ketones, carboxylic acids, and most carboxylic acid derivatives act as acids
Amines and amides act as bases
28
How do the definitions of nucleophile and electrophile differ from this of Lewis base and acid?
Nucleophilicity and electrophilicity are based on relative rates of reactions and are therefore kinetic properties. Acidity and basicity are measured by the position of equilibrium in a protonation or deprotonation reaction and are therefore thermodynamic properties
29
Rank the following molecules in order of increasing nucleophilicity: methoxide, t-butoxide, isopropanolate, ethoxide
t-butoxide < isopropanolate < ethoxide < methoxide
The four main determinants of nucleophilicity are charge (more negative = better nucleophile), electronegativity (more electronegative = worse nucleophile), steric hindrance (larger = worse nucleophile), and the solvent (protic solvents can protonate or hydrogen bond with the nucleophile, decreasing its reactivity). Each of these nucleophiles has the same attacking atom (oxygen), but differ in their bulkiness. The molecules with the least steric hindrance will be the more effective nucleophiles
30
How must the nucleophile and leaving group be related in order for a substitution reaction to proceed?
A substitution reaction will proceed when the nucleophile is a stronger base (more reactive) than the leaving group.
31
What trends increase electrophilicity?
Greater positive charge increases electrophilicity, and better leaving groups increase electrophilicity by making the reaction more likely to proceed.
32
What are some features of good leaving groups?
Good leaving groups can stabilize the extra electrons that result from heterolysis. Weak bases (the conjugate bases of strong acids) are good leaving groups. Resonance stabilization and inductive effects from electron-withdrawing groups also improve leaving group ability.
33
What are some characteristics of good oxidizing agents? List some examples of common oxidizing agents.
Good oxidizing agents have high affinity for electrons or have very high oxidation states. Examples include O2, O3, Cl2, permanganate (MnO4-), chromate (CrO4-), dichromate (CrO4 2-), and pyridinium chlorochromate. These compounds often contain a metal and a large number of oxygen atoms
34
What are some characteristics of good reducing agents? List some examples of common reducing agents.
Good reducing agents have low electronegativities and ionization energies or contain a hydride ion (H-). Examples include sodium, magnesium, aluminum, zinc, sodium hydride (NaH), calcium dihydride (CaH2), lithium aluminum hydride (LiAlH4), and sodium borohydride (NaBH4). These compounds often contains a metal and a large number of hydrides.
35
List the following carbon-containing compounds from most oxidized carbon to most reduced: methane, carbon dioxide, ketone, alcohol, carboxylic acid
most oxidized - Carbon dioxide, carboxylic acid, ketone, alcohol, methane - most reduced
36
What are the two reactive centers of carbonyl-containing compounds?
The two reactive centers are the carbonyl carbon, which is electrophilic, and the alpha hydrogens, which are acidic
37
What are the six steps for solving organic chemistry reactions?
1. Know your nomenclature
2. Identify the functional groups
3. Identify the other reagents
4. Identify the most reactive function group(s)
5. identify the first step of the reaction
6. Consider stereoselectivity
38
If there are no reaction conditions listed, what determines how the reaction will proceed?
If there are no reagents other than the reactants, then the properties of the functional groups on the reactants themselves (acid-base; nucleophile-electrophile) will determine the outcome
39
Which has a lower pKa: ethanol or p-ethylphenol? Why?
Phenols like p-ethylphenol have increased acidity due to resonance and the electron-withdrawing character of the phenol aromatic ring. Because p-ethylphenol is a stronger acid than ethanol, it will have a lower pKa.
40
Rank the following by decreasing boiling point: 1-pentanol, 1-hexanol, 1,6-hexanediol
1,6-hexanediol will have the highest boiling point; a molecule with two hydroxyl moieties can have more hydrogen bonding. The 1-hexanol boiling point will be next, with 1-pentanol having the lowest boiling point. 1-hexanol has a higher boiling point than 1-pentanol because the longer hydrocarbon chain has increased van Der Waals forces.
41
What will happen to primary and secondary alcohols, respectively, in the presence of strong oxidizing agents?
In the presence of strong oxidizing agents, primary alcohols are completely oxidized to carboxylic acids. Secondary alcohols can only be oxidized to ketones.
42
What is the product when 1-butanol is treated with PCC? With chromium trioxide?
Reacting 1-butanol with PCC results in the aldehyde, 1-butanal. Chromium trioxide is a stronger oxidizing agent that will produce the carboxylic acid, butanoic acid
43
What is the purpose of a mesylate or tosylate?
Mesylates and tosylates are used to convert an alcohol into a better leaving group. This is particularly useful for nucleophilic substitution because it increases the stability of the product. They can also be used as protecting groups because many reagents (especially oxidizing agents) that would react with an alcohol cannot react with these compounds
44
How can aldehydes or ketones be protected using alcohols?
Aldehydes or ketones can be reacted with two equivalents of alcohol or a dill to form an acetal or ketal. Acetals and ketals are less reactive than aldehydes and ketones (especially to reducing agents), and can thus protect the functional group from reacting. The acetal or ketal can then be reverted back to the carbonyl by catalytic acid.
45
How are quinones generally produced?
Quinones are produced by oxidation of phenols
46
How are hydroxyquinones produced?
Hydroxyquinones are produced by the oxidation of quinones, adding a variable number of additional hydroxyl groups
47
What chemical properties of ubiquinone allow it to carry out its biological functions?
Ubiquinone has conjugated rings that stabilize the molecule when accepting electrons. Additionally, the long alkyl chain in the molecule allows for lipid solubility, which allows the molecule to function in the phospholipid bilayer.
48
Given an alkane, an aldehyde, and an alcohol with equal-length carbon chains, which will have the highest boiling point? Why?
The alkane will have the lowest boiling point, followed by the aldehyde, and then the alcohol. The boiling point of the aldehyde is elevated by its dipole, but the boiling point of an alcohol is further elevated by hydrogen bonding.
49
Is the carbon of a carbonyl electrophilic or nucleophilic? Why?
The carbon in a carbonyl is electrophilic; it is partially positively charged because oxygen is highly electron-withdrawing.
50
What is one method for forming an aldehyde? A ketone?
aldehydes can be formed by the oxidation of primary alcohols, but can only be produced using weaker (and anhydrous) oxidizing agents like PCC - otherwise, they will oxidize fully to carboxylic acids. Ketones can be formed by the oxidation of secondary alcohols.
51
When an aldehyde or ketone is reacted with one equivalent of an alcohol, what occurs? What would be different if it were reacted with two equivalents in acidic conditions?
With one equivalent of alcohol, aldehydes and ketones will form hemiacetals and hemiketals, respectively. With two equivalents of alcohol, the reaction will run to completion, forming acetals and ketals, respectively.
52
When nitrogen or nitrogen-containing derivatives react with aldehydes and ketones, what type of reaction happens, and what function group is formed?
The reaction that occurs is a condensation reaction because a small molecule is lost, and also a nucleophilic substitution reaction. This reaction results in the formation of an imine (or, for nitrogen-containing derivates: oximes, hydrazones, or semicarbazones).
53
When HCN reacts with an aldehyde or ketone, what functional group is produced? Is the product stable?
When HCN reacts with an aldehyde or ketone, a cyanohydrin is produces, which is a stable product.
54
What functional group is formed when an aldehyde is oxidized? What are some common oxidizing agents that assist this reaction?
Oxidizing an aldehyde yield a carboxylic acid. Common oxidizing agents include KMnO4, CrO3, Ag2O, and H2O2
55
What functional group is formed when aldehydes and ketones are reduced? What are some common reducing agents that assist this reaction?
Reducing an aldehyde or ketone yields an alcohol. Common reducing agents include LiAlH4 and NaBH4
56
A chemistry student reacts butanone and butanal each with PCC and KMnO4. What are the expected products of each reaction?
Butanone reacts with neither PCC nor KMnO4 because ketones cannot be oxidized with common oxidizing reagent that cannot break the carbon-carbon bond. Butanal is oxidized by KMnO4 to form butanoic acid, but does not react with PCC, which is not a strong enough oxidant.
57
Why are the alpha-hydrogens of aldehydes and ketones acidic?
The alpha-hydrogens of aldehydes and ketones are acidic, or deprotonate easily, due to both inductive effects and resonance effects. The electronegative oxygen atom pulls electron density from the C-H bond, weakening it. Once deprotonated, the resonance stabilization of the negative charge between the alpha-carbon, carbonyl carbon, and electron-withdrawing carbonyl oxygen increases the stability of this form.
58
Which has a lower pKa: 3-pentanone or pentanal? Why?
The alpha-hydrogens of aldehydes are slightly more acidic than those of ketones due to the electron-donating characteristics of the second alkyl group in ketones. This extra alkyl group destabilizes the carbanion, which slightly disfavors the loss of the alpha-hydrogens in ketones as compared to aldehydes. Therefore, pentanal is a stronger acid than 3-pentanone and will have a lower pKa.
59
How does steric hindrance affect the relative reactivity of aldehydes and ketones?
Steric hindrance is one of the two reasons that aldehydes are slightly more reactive than ketones. The additional alkyl group gets in the way and makes for a higher-energy, crowded intermediate.
60
What are tautomers?
Tautomers are isomers that can be interconverted by the movement of a hydrogen and a double bond
61
Which tautomer of aldehydes and ketones is thermodynamically favored: keto or enol?
The keto form is thermodynamically favored
62
Which role does the enolate carbanion play in organic reactions: nucleophile, electrophile, oxidizing agent, or reducing agent?
Enolate carbanions act as nucleophiles
63
What is the kinetic enolate and the thermodynamic enolate? What conditions favor the formation of each?
The kinetic enolate is formed more rapidly but is less stable. This form has the double bond to the less substituted alpha-carbon. The kinetic form is favored by fast, irreversible reactions, such as with a strong, sterically hindered base, and lower temperatures.
The thermodynamic enolate is formed more slowly but is more stable. This form has the double bond being formed with the more substituted alpha-carbon. The thermodynamic form is favored by slower, reversible reactions, with weaker or smaller bases, and higher temperatures.
64
Identify the nucleophile and electrophile in the aldol condensation.
In the aldol condensation reaction, the enolate carbanion (the deprotonated aldehyde or ketone) acts as the nucleophile and the keto form of the aldehyde or ketone acts as the electrophile.
65
What is a retro-aldol reaction? What conditions favor retro-aldol reactions?
A retro-aldol reaction is the reverse of an aldol reaction where instead a bond between the alpha and beta-carbons of a carbonyl is broken. This can be favored by the addition of base and heat. In this reaction, a bond between the alpha and beta-carbons of a carbonyl is broken
66
The aldol condensation can be classified under many categories of reactions. List some of these reaction types, and provide a short description of each.
An aldol condensation is a condensation reaction, in which two molecules are joined to form a single molecule with the loss of a small molecule; a dehydration reaction, in which a molecule of water is lost; and a nucleophile-electrophile reaction, in which a nucleophile pushes an electron pair to form a bond with an electrophile.
67
What causes the relatively high acidity of carboxylic acids?
Carboxylic acids are particularly acidic due to the electron-withdrawing oxygen atoms in the functional group and the high stability of carboxylate anion, which is resonance stabilized by delocalization with two electronegative oxygen atoms.
68
Between a monocarboxylic acid, a dicarboxylic acid, and a dicarboxylic acid that has been deprotonated once, which will be the most acidic? Why?
A dicarboxylic acid would be the most acidic, as the second carboxyl group is electron-withdrawing and therefore contributes to even higher stability of the anion after loss of the first hydrogen. However, a mono carboxylic acid is more acidic than a deprotonated dicarboxylic acid because the carboxylate anion is electron-donating and destabilizes the product of the second deprotonation step, resulting in decreased acidity.
69
What effects do additional substituents have on the acidity of carboxylic acids?
Electron-withdrawing substituents make the anion more stable and therefore increase acidity. Electron-donating substituents, on the other hand, destabilize the anion, causing the carboxylic acid to be less acidic. The closer the substituent is to the carboxylic acid on the molecule, the stronger the effect will be.
70
What is the nucleophile used to form an amide in an acyl substitution reaction and what is the name of the cyclic form?
Formed by reaction with ammonia (NH3) or an amine
Name of cyclic form: lactam
71
What is the nucleophile used to form an ester in an acyl substitution reaction and what is the name of the cyclic form?
Formed by reaction with alcohol
Name of cyclic form: lactone
72
What is the nucleophile used to form an anhydride in an acyl substitution reaction and what is the name of the cyclic form?
Formed by reaction with another carboxylic acid
Name of cyclic form: anhydride
73
Briefly describe the mechanism of nucleophilic acyl substitution reactions
Nucleophile acyl substitution is the substitution of an attacking nucleophile for the leaving group of an acyl compound, which includes carboxylic acids, amides, esters, and anhydrides. The nucleophile attacks, opening the carbonyl and forming a tetrahedral intermediate. The carbonyl then reforms, kicking off the leaving group. This reaction is favored by acidic or basic conditions.
74
What is the result when butanoic acid is reacted with sodium borohydride? With lithium aluminum hydride?
Sodium borohydride is not strong enough to reduce carboxylic acids so there will be no reaction. Lithium aluminum hydride, however, is strong enough to reduce carboxylic acids to primary alcohols - producing 1-butanol
75
Under what conditions will a carboxylic acid spontaneously decarboxylate?
1,3-dicarboxylic acids will spontaneously decarboxylate when heated, due to the stable cyclic intermediate step.
76
For the carboxylic acid derivative of amides, list the relevant nucleophile that reacts with a carboxylic acid to generate the amide, the amide's suffix, and the name of the amide in cyclic form.
Formed from -COOH by: ammonia or an amine
Suffix: -amide
Cyclic naming: lactam
77
For the carboxylic acid derivative of esters, list the relevant nucleophile that reacts with a carboxylic acid to generate the ester, the amide's suffix, and the name of the ester in cyclic form.
Formed from -COOH by: an alcohol
Suffix: -oate
Cyclic naming: lactone
78
For the carboxylic acid derivative of anhydrides, list the relevant nucleophile that reacts with a carboxylic acid to generate the anhydride, the amide's suffix, and the name of the anhydride in cyclic form.
Formed from -COOH by: another carboxylic acid
Suffix: anhydride
Cyclic naming: anhydride
79
What is the definition of a condensation reaction?
A condensation reaction is one in which two molecules are joined with the loss of a small molecule. In all of the examples in this section, the small molecule lost was water
80
Rank the following molecules by decreasing reactivity to OR-: acetamide, acetic anhydride, and ethyl acetate
Anhydrides are the most reactive to nucleophiles. followed by esters, and then amides. Therefore, acetic anhydride will be the most reactive, followed by ethyl acetate, and finally acetamide
81
What is responsible for the increased rate of hydrolysis in ß-lactams?
ß-lactams are susceptible to hydrolysis due to the high level of ring strain, which is due to both torsional strain (eclipsing interactions) and angle strain (deviation from 109.5˚)
82
What properties account for the differences in reactivity seen between anhydrides, esters, and amides with nucleophiles?
Electronic effects like induction have some effect on the reactivity of the carbonyl in these three functional groups. Differences in resonance also explain the increased reactivity of anhydrides, in particular. Steric effects could also be significant, depending on the specific leaving group present.
83
In the formation of an amide from ammonia and an anhydride, what serves as the nucleophile? The electrophile?
Ammonia acts as the nucleophile. One of the carbonyl carbons of the anhydride serves as the electrophile.
84
What are the products of the transesterification reaction between isopropyl butanoate and ethanol?
Transesterification is the exchange of one esterifying group for another in an ester. This reaction requires an alcohol as a nucleophile. In this case, the ethyl group of ethanol will replace the isopropyl group of isopropyl butanoate, resulting in ethyl butanoate and 2-propanol.
85
How do strongly acidic and strongly basic conditions catalyze the hydrolysis of an amide?
Strongly acidic conditions catalyze amide hydrolysis by protonating the oxygen in the carbonyl. This increases the electrophilicity of the carbon, making it more susceptible to nucleophilic attack. Strongly basic conditions greatly increase the concentration of OH-, which can act as a nucleophile on amide carbonyls.
86
What makes glycine unique among the amino acids?
All amino acids, except glycine, have chiral alpha-carbons. Because the R group of glycine is a hydrogen atom, it is not chiral and therefore is not optically active.
87
Amino acids are amphoteric. What does this mean? What functional groups give amino acids this characteristic?
Amphoteric molecules can act as acids or bases. Carboxylic acids give amino acids their acidic properties because they can be deprotonated. Amino groups give amino acids their basic properties because they can be protonated.
88
How are peptide bonds formed and cleaved?
Peptide bonds are formed by a condensation reaction, in which water is lost, and cleaved hydrolytically by strong acid or base
89
Why is the C-N bond of an amide planar?
The C-N bond of an amide is planar because it has partial double-bond character due to resonance. Double bonds exist in a planar conformation and restrict movement.
90
What are the four reactants in the Strecker synthesis of an amino acid?
An aldehyde, ammonium chloride (NH4Cl), and potassium cyanide (KCN) are used to make the aminonitrile; water is used to hydrolyze the aminonitrile to form the amino acid
91
What are the reaction types used in the Strecker synthesis?
Stecker synthesis is a condensation reaction (formation of an amine from a carbonyl-containing compound and ammonia, with loss of water), followed by nucleophilic addition (addition of the nitrile group), followed by hydrolysis
92
What are the four main reactants in the Gabriel synthesis of an amino acid?
Gabriel synthesis begins with potassium phthalimide and diethyl bromomalonate, followed by an alkyl halide. Water is then used to hydrolyze the resulting compound to form the amino acid. While acids and bases are used at various times as catalysts, they are not main reactants.
93
What are the reaction types used in the Gabriel synthesis?
Gabriel synthesis proceeds through two SN2 reactions, hydrolysis, and decarboxylation.
94
What characteristics make inorganic phosphate so useful for energy transfer biologically?
Inorganic phosphate contains a very negative charge. When bonded to other phosphate groups in a nucleotide triphosphate, this creates repulsion with adjacent phosphate groups, increasing the energy of the bond. Further, inorganic phosphate can be resonance-stabilized.
95
What is an organic phosphate?
Organic phosphates are carbon-containing molecules with phosphate groups; the most common examples are nucleotides, like those in DNA, ATP, or GTP
96
What characteristics of phosphoric acids make them good buffers?
The three hydrogens in phosphoric acid have very different pKa values. This allows phosphoric acid to pick up or give off protons in a wide pH range, making it a good buffer over most of the pH scale.
97
What does infrared (IR) spectroscopy measure? What is IR spectroscopy generally used for?
IR spectroscopy measures absorption of infrared light by specific bonds that vibrate. These vibrations cause changes in the dipole moment of the molecule that can be measured. Once the bonds in a molecule are determined, one can infer the presence of a number of functional groups to determine the identity of the molecule.
98
What two peaks would you expect to see in the IR spectrum, of a carboxylic acid?
A carboxylic acid would have a broad O-H peak around 2800-3200 cm^-1 and a sharp carbonyl peak at 1700-1750 cm^-1
99
Which of the following molecules are detectable by UV spectroscopy: propane, propene, and propanone?
Conjugates systems and other molecules with π or nonbonding electrons can give absorbances on a UV spectroscopy plot. Therefore, propane would not be detectable, but propene and propanone would.
100
In UV spectroscopy, what is the HOMO? What is the LUMO? How are they related to absorption wavelength?
HOMO is the highest occupied molecular orbital; LUMO is the lowest unoccupied molecular orbital. The smaller the difference in energy between the two, the longer the wavelengths that can be absorbed by the molecule
101
What does nuclear magnetic resonance (NMR) spectroscopy measure? What is NMR spectroscopy generally used for?
NMR measures alignment of the spin of a nucleus with an applied magnetic field. It is most often used for identifying the different types of magnetic environments of protons in a molecule, which allows us to infer the connectivity (backbone) of a molecule.
102
What are the units for chemical shift on a standardized NMR spectrum?
The units for chemical shift with a standardized NMR spectrum are parts per million (ppm).
103
What does it mean for a proton to be deshielded? How does this affect its peak in NMR spectroscopy?
Deshielding occurs in molecules that have electronegative atoms that pull electron density away from the hydrogens being measured. This results in a downfield (leftward) shift of the proton peak
104
What is spin-spin coupling?
Spin-spin coupling occurs when two protons close to one another have an effect on the other's magnetic environment. This results in the splitting of peaks into doublets, triplets, or multiplets, depending on the environment
105
What must be true about the two solvents used for an extraction to work?
The two solvents must be immiscible and must have different polarity or acid-base properties that allow a compound of interest to dissolve more easily in one than the other.
106
When doing an extraction, would it be better to do three extractions with 10 mL of solvent. or one extraction with 30 mL?
It is better to do three washes with 10 mL than to do one with 30 mL; more of the compound of interest would be extracted with multiple sequential extractions than one large one.
107
Would acid dissolve better in an aqueous acid or aqueous base? Why?
Acid dissolves better in an aqueous base because it will dissociate to form conjugate base and, being more highly charged, will become more soluble. Note that like dissolves like applies to polarity; acids and bases dissolve more easily in solutions with the opposite acid-base characteristics.
108
Distillation separates compounds based on what property?
Distillation takes advantage of differences in boiling points in order to separate solutions of miscible liquids.
109
If we are given a solution of ether, with a boiling point of 308 K, and methylene chloride, with a boiling point of 313 K, which type of distillation should be used to separate them?
A solution of ether and methylene chloride, which have very close boiling points, can be separated by using fractional distillation.
110
If we are given a solution of bromobenzene, with a boiling point of 156˚C, and camphor, with a boiling point of 204˚C, which type of distillation should be used to separate them?
Vacuum distillation would be the best technique to separate two chemicals with such high boiling points because the decreased ambient pressure will allow them to boil at a lower temperature.
111
What properties of molecules do thin-layer chromatography, paper chromatography, and standard column chromatography take advantage of to separate compounds?
Each of these methods separates compounds using charge and polarity.
112
What are the three specialized types of column chromatography? What does each use in order to separate the sample?
Ion-exchange: the column is given a charge, which attracts molecules with the opposite charge.
Size-exclusion: Small pores are used; smaller molecules are trapped, while larger molecules pass through the column
Affinity: specific receptors or antibodies can trap the target in the column; the target must then be washed out using other solutions
113
In what ways is gas chromatography distinct from all of the other techniques we have discussed?
As the name suggests, gas chromatography is simply the same technique of mobile and stationary phases performed with a gaseous fluent (instead of liquid). The stationary phase is usually a crushed metal or polymer
114
What is the major historical distinction between HPLC and column chromatography? What is the major distinction now?
Historically, HPLC was performed at high pressures, whereas column chromatography uses gravity to pull the solution through the column. Now, HPLC is performed with sophisticated and variable solvent and temperature gradients, allowing for much more specific separation of compounds than column chromatography; high pressures are no longer required
