ochem 2 Flashcards
A bond with no ___ will not be detected by IR
dipole
The vibration frequency is determined by :
strength of the bond
molecular weight of the bonded atoms
Describe the similarities between vibrational frequency in IR spectroscopy and a mass- spring system. How do bond strength and molecular weight impact frequency?
We can compare a vibrating bond (in only one dimension, ignoring swaying motions) to a mass on a spring. The bond roughly follows Hooke’s Law and either atom can be compared to the mass. A stronger bond would be analogous to a spring with a larger spring constant (k), and a weaker bond would be analogous to a spring with a smaller spring constant. The molecular weight of the atom is analogous to the size of the mass on the spring. Following the principles dictated by Hooke’s Law, a stronger spring (larger k) increases oscillation frequency according to: f = (1/2pi) √(k/m). A weaker bond would therefore vibrate at a lower frequency. A smaller atom is associated with a higher vibrational frequency and a larger atom is associated with a lower vibrational frequency.
What is the IR absorbance for a carbonyl (C=O)
1700 cm-1 (sharp,deep)
What is the IR absorbance for an alcohol?
3300 cm-1 (broad, separate from CH)
What is the IR absorbance of a saturated alkane, CH?
2800 cm-1 sharp, deep
What is the IR absorbance of carboxylic acid?
3000 cm-1 (broad, overlaps CH)
What is the IR absorbance of an amine?
3300 cm-1 , (broad shallow)
What is the IR absorbance of an amide?
3300 cm-1 (broad, deep)
What is the IR absorbance of a nitrile?
2250 cm-1 (sharp, deep)
Molecules containing only ___ bonds show low or no UV absorbance in UV spectroscopy
single
What type of bonds absorb UV strongly?
double and triple bonds
___ absorb UV even more strongly than do isolated double or triple bonds
conjugated systems
The greater the degree of conjugation, the farther ___ the species will absorb
right (higher wavelength)
The height of each peak gives the relative ___ of that fragment in mass spectrometry
abundance
What is the parent peak of a mass spectrometry?
represents the original molecule minus one electron
What is the base peak of a mass spectrometry?
is the most common fragment and is usually the most stable fragment generated. It is defined as existing at 100% relative abundance. In other words, it will be the highest peak on the spectra and the height of all other peaks will be a function of how abundant that fragment is compared to the base peak.
an atom must have either a __ atomic number or a __ mass number to register on an NMR (or MRI)
odd, odd
What do the peaks in H-NMR spectras mean?
represent all of the hydrogens in a molecule that share an indistinguishable chemical environment
What does the area under the peak of an H-NMR represent?
representation of the number of hydrogens accounted for by that peak
absorbance range in a H-NMR spectra is__
0-12 ppm
12 ppm in a H-NMR is __
downfield= deshielded
0 ppm is __ in a H-NMR spectra
upfield = shielded
What is spin splitting in HNMR spectras?
the presence of “neighbors” (non-equivalent hydrogens attached to a neighboring carbon) causes splitting of the peak. The peak for a set of equivalent hydrogens will be split into exactly n + 1 sub-peaks
What is the main difference between H-NMR and C13-NMR?
in C13-NMR, there is no spin-spin splitting (all peaks are singlets) No integration (area under curve does not indicate the relative number of carbons)
What is the absorbance range of C13-NMR?
0-220 ppm ; 220 ppm is downfield = deshielded, 0 ppm is upfield = shielded
What is the C13 absorbance of C–C bond?
0-50 ppm
What is the C13 absorbance of a C–O bond ?
50-100
What is the C13 absorbance of a C=C?
100-150
What is the C13 absorbance of a C=O bond?
150-200
What is the purpose of extraction?
separate two compounds, or to remove a desired product from a reaction mixture. Two immiscible liquids, a polar (aqueous) and a non-polar (organic) are used. The separation depends on the target molecule having differential solubility in the two solvents.
What can improve separation?
repetition
fractional extraction (extracting smaller amounts)
addition of an acid to protonate the product
addition of a base to deprotonate the product
When looking at an extraction problem on the MCAT, you can assume that the nonpolar product is __ and the polar product is located __
on top, on the bottom layer
Things to avoid during extractions
mixing too vigorously
reactive solvents
high boiling points
What is gravity filtration?
physical separation of a solid from a liquid by passing through filtration paper
What is vacuum filtration? How does it differ from gravity filtration?
vacuum filtration is performed with a Buchner funnel. A vacuum is created inside of the flask which creates suction to pull filtrate through the filter paper (faster than gravity filtration)
What is simple distillation?
Heat the mixture in a flask; the liquid with the lower boiling point evaporates first, enters a collecting arm, cools, and drops into a collecting flask. Cooling the condensing arm or placing the collecting flask on ice can improve the separation process.
What is fractional distillation?
A fractionating column is placed between the heating flask and the condensing arm. The mixture is heated to slightly above the boiling point of the more volatile liquid. The gas rises through a column of glass beads or metal shards. This causes any impurities in the vapor (i.e., molecules from the liquid with the higher boiling point) to condense and fall back into the flask, resulting in a better separation. The more volatile component will be above its boiling point and therefore will not condense. This approach allows separation of compounds with boiling points less than 25 ̊C apart.
What is vacuum distillation?
The air inside the apparatus is evacuated to create a vacuum
When would you want vacuum distillation?
when you want to dramatically lower the boiling point of the substance, allowing for more manageable temps
For chromatography in general, the first substance “out”, will be ___
LEAST polar
What is Paper or Thin layer chromatography?
paper chromatography uses paper as the stationary phase
TLC uses manufactured glass or plastic sheets coated w silica, alumina etc.
What is Rf in TLC?
distance traveled by component/ distance traveled by solvent
If Rf = 0.9, is the substance in question polar or non-polar?
a number close to one means that the polarity of the component is similar to that of the solvent. Because non-polar solvents are normally used, the component is therefore relatively non-polar.
Why are smears present on TLC or paper chromatography?
solvent is unable to dissolve and carry the components as it moves up the paper via capillary action . This may because too much sample was spotted onto paper, also if there is too big of a difference in polarity between sample and solvent.
What is the difference between column chromatography and ion-exchange chromatography?
column chromatography - mixture passed through column of charged glass beads (polar)
ion-exchange - column is coated with cations or anions. mixture is passed through and oppositely - charged ions adhere to column. target molecules can then be eluted by washing with a salt solution
What is affinity chromatography?
used to isolate specific molecule based on very SPECIFIC AFFINITY or binding interaction
What is gas chromatography?
a liquid is used as a stationary phase. mixture is dissolved into a heated gas and then passed through the liquid
exit based on 1) boiling point 2) polarity
What causes one peak to be higher than another on a gas chromatograph?
The height of the peak in a gas chromatograph is relative to the abundance of that component
How does recrystallization work?
The desired product is dissolved in the minimum amount of hot solvent necessary. solution is cooled as slowly as possible (impurities have lower melting points, so pure product will remain solid and impurities are solution)
___ and __ melting points indicate better purity.
higher, sharper
the carbonyl bond is __ and __ than an alkene
shorter and stronger
Why is a C=O bond stronger than a C=C bond?
because oxygen has a smaller atomic radius, there is more room for pi overlap
What are the 5 key features of carbonyls?
- partial pos charge on the carbon (good electrophile)
- alpha hydrogens (acidic)
- electron donating/withdrawing groups (donating groups decrease reactivity of carbonyl carbon/ withdrawing groups increase reactivity)
- steric hindrance (bulky substituents decerase reactivity)
- planar stereochemistry - (can be attacked from either side)
Why are hydrogens on the alpha carbon of the carbonyl acidic?
resonance stabilization of the conjugate base, when there are 2 carbonyls separated by a single carbon the hydrogen in the middle carbon are EVEN MORE ACIDIC
What is an aldehyde?
any compound containing a carbonyl with one or more hydrogen substituents on the carbonyl carbon. A ketone is any compound containing a carbonyl with two carbon substituents on the carbonyl carbon.
aldehydes are named with the __ ending
-al
ketones are named with the ___ ending
-one
aldehydes and ketones can act as ___ but not as __
H-bond recipients, not H-bond donors
The main function of aldehydes and ketones is __
being an electrophile (carbonyl carbon)
aldehydes and ketones undergo nucleophilic ___
carboxylic acids/amides/esters/anhydrides undergo nucleophilic ___
addition, substitutions
strong bases never make good ___
leaving groups
When is the only time where ketones/aldehydes can act as a H-bond donor?
in 1,3-dicarbonyl compounds
the enol form acts as the H-bond donor in intramolecular hydrogen bonding
What is a hemiacetal/hemiketals?
one OR substituent and one OH substituent
What is a acetal/ketal?
has 2 -OR substituents
What are the steps of forming a hemiacetal and acetal?
- acid protonates the carbonyl oxygen
- alcohol attacks carbonyl, double bond moves up to oxygen
- base extracts proton from original alcohol (becomes hemiacetel)
- acid protonates alcohol to make H2O (good leaving group)
- lone pair from ether oxygen forms double bond, kicks out water
- alcohol attacks carbonyl, double bond electrons move up to oxygen
- base deprotonates new ether (get acetal)
What are the steps to protect ketones/aldehydes from reaction?
(use nucleophile or base to turn into acetal or ketal)
(diols with at least 2 carbons works best, 2 equivalents of alcohol)
1. one end of the diol acts as the nucleophile, attacking the carbonyl
2. the other end of the diol acts as the “second equivalent of alcohol”
3. acidic conditions will return the acetal/ketal to original
steps of halogenation of an aldehyde or ketone
- a base abstracts an alpha hydrogen, leaving a carbanion
2. the carbanion attacks a diatomic halogen
What is aldol condensation?
the condensation of one aldehyde or ketone with another aldehyde or ketone
what are the steps of aldol condensation?
- a base abstracts an alpha hydrogen, creating a carbanion
- the carbanion will attack any carbonyl carbon in the solution
- the oxygen is protonated to form an alcohol
Is an α-β unsaturated carbonyl a base, a nucleophile, or an electrophile?
electrophile
What are the steps of the α-β unsaturated carbonyl electrophilic addition mechanism?
1) With the double bond between the alpha and beta carbons, the nucleophile attacks the beta carbon, pushing the double bond over one carbon and forcing the C=O electrons up onto the oxygen.
2) With a carbocation on the beta carbon, the nucleophile simply attacks the beta carbon directly.
carboxylic acids are named with the ___ ending
-oic
What are physical properties of carboxylic acids?
very high boiling points
soluble in water (without long carbon chains)
carboxylic acids can hydrogen bond how many times?
twice
What is the reaction for the nucleophilic attack of carboxylic acids?
RCOOH + H2O→ RCOOH2+ + Nu:- → RCONu + H2O
What is the mechanism of the decarboxylation of a beta-keto carboxylic acid?
- a base extracts a proton from the OH
- double bond forms between alpha and beta carbon
- co2 leaves
- enol forms keto
What is the reaction of esterification of a carboxylic acid?
RCOOH + ROH → RCOOR + H2O
acid chlorides are named with the ___ ending
-oyl chloride
What are the 3 reagents that produce acid chlorides?
PCl3, PCl5 and SOCl2
___ are the most reactive of the carboxylic acid derivatives
acid chlorides
What is an anhydride?
compound with two acyl groups connected to one another by a single oxygen
how do you name anhydrides?
replace the “oic” ending with “oic anhydride”
ie benzoic acid -> benzoic anhydride
What are the main properties of anhydrides?
excellent electrophiles
What is an amide?
any compound containing a carbonyl with an amine substituent on the carbonyl carbon
How do you name amides?
replace “oic” ending with “amide”
ie benzoic acid -> benzamide
Are amides reactive?
not really, NH2 is NOT a good leaving group
__ and __ amides can hydrogen bond
primary and secondary
lone pair on the amide Nitrogen resonates with the carbonyl double bon, giving both C-O and C-N bonds ___. this prevents rotation
double bond character
Are amines or amides more basic?
amines (more electron dense)
What is the function of Hofmann Degradation?
allows you to add an amine to a tertiary carbon (turns amide to an amine)
What is an ester?
compound containing a carbonyl with an –OR group substituted on the carbonyl carbon.
How are esters named?
–oate ending, with the R portion of the –OR group named and placed in front of the name, as is methyl pentanoate
esters are H-bond ____, not ___
recipients, not donors
What is the reaction for transesterification?
Reaction of an existing ester with an alcohol, creating a different ester. Also requires acid catalysis
RCOORa + RbOH → RCOORb + RaOH
What is saponification?
Hydrolysis of an ester to yield an alcohol and the salt of a carboxylic acid
What are the steps of saponification?
1) The hydroxide ion (NaOH or KOH) attacks the carbonyl carbon and pushes the C=O electrons up onto the oxygen.
2) The electrons collapse back down and kick off the –OR group.
3) Either the –OR group, or hydroxide ion, abstracts the carboxylic acid hydrogen, yielding a
carboxylate ion. This associates with the Na+ or K+ in the solution to form “soap.”
What is acetoacetic ester synthesis?
formation of a ketone from a beta-keto ester
What are the steps for the acetoacetic ester synthesis mechanism?
1) A base abstracts the acidic alpha hydrogen, leaving a carbanion.
2) The carbanion attacks an alkyl halide (R-X), resulting in addition of the –R group to the alpha
carbon.
3) Hot acid during workup causes loss of the entire –COOR group.
Rank the following leaving groups from best to worst; NH2, Cl, OCOR, OR, OH
-Cl > -OCOR > -OH > -OR > -NH2
the stability of carboxylic acid derivatives decreases as the ___
better the leaving group
Amines can act as __ or __
bases, nucleophiles
Rank the basicity of primary, secondary, tertiary amines and ammonia in increasing basicity
ammonia, tertiary amine, primary amine, secondary amine
ammonium acts as a ___
electrophile
Which functional group forms stronger hydrogen bonds, alcohols or amines?
alcohols (O is further away from H than N)
amine nomenclature
1) Name the alkane to which the N is attached (e.g., propane)
2) Add “amine” in place of the “e” on the end of “ane” (e.g., propanamine). It is also acceptable to separate the substituent name (e.g., propyl amine)
3) If the amine is secondary, the longest chain is included in the name as indicated above. The other chain is added at the beginning, proceeded by the letter “N-“ (e.g., N-ethylpropanamine)
4) If the amine is tertiary or quaternary, add additional substituents to the front of the name in alphabetical order, all with the prefix N- included (e.g., N,N-diethylpropanamine, or N-ethyl-N- methylpropanamine, or N,N-dimethyl-N-ethylpropanamine)
What is the reaction of the synthesis of alkyl amines?
NH3 + CH3Br → NH2CH3 + HBr
1) Ammonia acts as a nucleophile, attacking the alkyl halide via SN2 and kicking off the halide ion.
2) The halide ion acts as a base, abstracting a hydrogen to quench the charge on the nitrogen.
What is the purpose of Gabriel synthesis?
Formation of a primary amine from a primary alkyl halide; avoids the side products of alkyl amine synthesis.
What are the steps of Gabriel synthesis?
The phthalimide ion, a reactive species with a full negative charge on the nitrogen, acts as a nucleophile, attacking the alkyl halide via SN2.
How do you reduce nitro groups, nitrile groups, imines, and amides?
nitro - LiAlH4, NaBH4, and H2/catalyst with pressure.
nitrile - (LiAlH4 mainly) and all above
imines - (NaBH3CN or H2 and a Catalyst) and all above
amides - reduce to primary amine via LiAlH4 ONLY
What are the steps of the formation of enamines and imines?
Amines add to aldehydes and ketones to form imines and enamines
1) The amine acts as a nucleophile, attacking the electrophilic carbonyl carbon.
2) The oxygen is protonated twice, creating the good leaving group water.
3) A base abstracts a hydrogen from the nitrogen and kicks off water in an E2 mechanism. This
forms either an imine or an enamine (depending on the substitution pattern of the nitrogen).
primary amines yield __
imines
secondary amines yield __
enamines
tertiary amines yield __
do not react
