CHEM 222L Final Exam
Eluent
"carrier" portion of mobile phase
What is oxidation state of C=O in camphor?
+2
Functional groups on fluorene and fluorenone?
-H, =O
Which chemical species acts as Bronsted base? Which acts as a Lewis base?
-OH is both. Accepts proton from HCl H2O Donates electron to methyl salicylate form hydroxyl bond
What is oxidation state of C-OH carbon in isoborneol?
0
Typical mp range of pure compound?
0.5 to 2.0°C
A mixture of fluorene, fluorenone and fluorenol is examined by TLC and gives the following Rf values: 0.3, 0.5, 0.8. Identify each spot and rationalize the relative Rf values.
0.8 is fluorene, 0.5 is fluorenone, 0.3 is fluorenol. Fluorenol is an alcohol, so it is more polar than the ketone fluorenone, giving it a lower Rf value.
How many asymmetric C atoms in isoborneol
1
Purposes of TLC analysis?
1) assess progress of a reaction 2) assess purity of a compound 3) provide information about identity of a compound
Synthesis of salicylic acid: role of NaOH
1. deprotonates phenol group of methyl salicylate 2. attacks carbonyl carbon of methyl ester group to give disodium salicylate
What do you do if crystals don't form?
1. scratch with glass stir rod 2. try adding seed crystal 3. evaporate some solvent
Potassium salicylate + sulfuric acid rxn
2RCOO⁻K⁺ + H₂SO₄ → 2RCOOH + K₂SO₄
What might happen if the bulb of the thermometer is above the branch in the distillation head?
Above: will read as too low temperature, will possibly reach BP of compound with higher BP → imperfect separation
What happens when HCl is added to flask 1? (purpose of HCl)
Acidifies sodium benzoate and neutralizes remaining HCl. Produces benzoic acid (insoluble in water), which is filtered
How would you separate mixture of 4-methylbenzoic acid (p-toluic acid) and anthracene (organic)?
Add TBME and NaOH(aq) to create aqueous and organic layers, use separatory funnel. Aqueous: add HCl to react away NaOH, filter Organic: wash with NaCl, dry with CaCl2, evaporate TBME
Distinguish aldehyde from ketone using IR
Aldehyde has C-H peak at 2900-2800
Organic solvent in face
Alert TA and lab mates, take off glasses, use eyewash for 10-15min
If student drops beaker
Alert TA, use dust-pan and brush, dispose in broken glass container
Rank all functional groups from most to least polar.
Amide > Acid > Alcohol > Amines > Ketone ~ Aldehyde, esters > Ethers, halocarbons > Aromatic hydrocarbons > Alkenes > Alkanes
Why is oxone preferable to bleach for oxidation of isoborneol to camphor?
Bleach has widely varying concentrations and does not generate consistent yield
Explain electrophilicity of the carbonyl carbon in methyl salicylate
Bonded to two oxygen atoms (both are more electronegative), making the carbonyl carbon more electropositive
Synthesis of benzyhydrl-1-propyl ether: what species is the electrophile in this reaction?
C carbocation linking aromatic groups
Distinguish carboxylic acid from ketone using IR
CA has C-O peak at 1300-1000
Given that compound A can be recrystallized from CH2Cl2 and ethyl acetate, which would you choose and why?
CH2Cl2 is a carcinogen. Choose EtOAc: only hazard is flammability
Oxidation states of CH₄, CH₂O, H2CO₃
CH4: -4 CH2O: 0 H2CO3: +4
Name three common drying agents
CaCl2, Na2SO4, magnesium sulfate
Purpose of CaCl2 pellets?
Chemical drying, remove last remnants of dissolved water in organic solution
Difference between chemical hazard and handling precaution, and example of each
Chemical hazard: qualities of chemical reagent (flammable, skin irritant) Handling precaution: actions of experimenter (use fume hood, use gloves)
What the features of a good drying agent?
Chemically stable/chemically inert, high capacity, high efficiency
In oxidation rxn, what serves as catalyst?
Cl-
TLC use to judge completeness of reaction
Compare TLC of product to TLC of starting material (and model product if possible)
Why does 1-propanol give higher yields than does cyclohexanol?
Cyclohexanol is a bulky nucleophile
How does TLC reveal purity of given compound?
Different compounds form different spots on TLC plate b/c move diff distances If more than one compound present, will see more than one spot on TLC plate
What type of intermolecular forces are present in benzoic acid?
Dipole-dipole interactions (-OH group)
Intermolecular interactions in sample of hexane
Dispersion
What makes ethyl acetate a useful solvent for this reaction?
EtOAc can dissolve the substrate without dissolving oxone and NaCl so that the reaction can take place at the interface of two phases. Also, it makes the reagents easier to be removed.
Camphor, ethyl acetate, water and sodium chloride(aq) are in a separatory funnel. Where is each?
EtOAc: upper layer, contains camphor H2O: lower layer, contains NaCl
What hazard do most organic solvents share?
Flammable
What hazard do ethyl acetate, camphor, and isoborneol share in common?
Flammable solid
Imagine that the TLC of flask 3, following column chromatography to separate fluorene and fluorenone, shows two spots with Rf 0.4 and 0.6. Identify the spots and comment on the success of separation procedure.
Fluorenone is 0.4, fluorene is 0.6 Unsuccessful separation
Relative Rf of flourene and fluorenone
Fluorenone is more polar (ketone), has lower Rf value than fluorene (alkene/aromatic)
Why does separation with NaOH work?
Good use of pKa values, NaOH is stonger base than sodium salicylate so reaction proceeds forward
At the end, how is excess basic solvent removed?
HCl is added, neutralizes NaOH
Why should one avoid making acid and base solutions in graduated cylinders?
Heat of dissolution will cause expansion (up and out)
Bond order and absorption of IR radiation
High bond order = absorb higher frequency IR radiation
Rank polarity of following solvents (high, intermediate, low): water, hexane, ethyl acetate, toulene, acetone, ethanol
High: Water Inter: ethanol, acetone, ethyl acetate Low: hexane, toulene
Main hazard with strongly basic solutions
Highly corrosive to skin and eyes
Hirsch vs Buchner funnel choice
Hirsch: a little bit of product Buchner: a lot of product
Why is the benzhydryl cation formed preferentially? (how does this work)
It is stabilized by resonance, double bonds move around (one goes onto C-C next to ether to start)
Two methods for IR prep for solid
KBr pellet Thin solid film
What the active oxidizing agent in Oxone?
KHSO₅
Impact of higher melting or high boiling impurity or solvent on mp of organic compound
Lower mp and broaden range
If salicylic acid is wet with water, what is the effect on mp?
Lowers mp, broadens range (disruption of crystalline structure)
Why might you get a % recovery of salicylic acid way under 100% (say 40%)?
Might not have added enough HCl, all salicylate didn't reprotonate to salicylic acid, reducing % recovery
Solvent better for recrystallization of 1,4-dimethoxybenzene: methanol, water, or a mixture?
Mixed solvent. Methanol dissolves the solute, water induces crystallization
column chromatography mobile and stationary phase
Mobile: solvent, analogous to TLC developing solution Stationary: alumina sand Al₂O₃
Order of elution in column chromatography: anthracene, 2-napthol, benzaldehyde, benzoic acid
Most polar will elute last Anthracene > benzaldehyde > 2-napthol > benzoic acid
NaOH + C6H5COOH (benzoic acid)
Na+C6H5COO- + H2O sodium benzoate and water
NaOH + HCl
NaCl + H2O
If Rf of starting material is 0.2 in 100% pentane, what change would you make to the solvent and why?
Need more polar solvent, maybe do 50/50 with ethyl acetate. Must compete with polarity of silica gel
Excess TBME and NaOH disposal
Neutralize with 5% acetic acid, organic waste container
Excess NaOH disposal
Neutralize with 5% acetic acid, wash down sink
Excess organic solvent contaminated with mineral acid
Neutralize with sodium bicarbonate, dispose in organic waste
If IR spectrums are identical, are two compounds identical?
No 1. Only detects functional groups 2. Doesn't detect stereochemistry (could be enantiomers or diastereomers) 3. IR can only detect polarizable bonds
(Exp 1) Imagine performing the experiment with sodium bicarbonate instead of sodium hydroxide. Will the experiment work?
No: pKa of NaHCO3 disfavors formation of sodium benzoate
What species is the nucleophile? (synthesis of benzyhydryl-1-propyl-ether)
O of 1-propanol
Why does one need to heat the reactants in this reaction?
OH- won't be able to attack the carbonyl carbon without heat
Excess TBME disposal
Organic waste container
Which has the higher vapor pressure, pentane or hexane? How does this affect BP?
Pentane (57.90kPa) > Hexane (17.60kPA), means that pentane has lower BP
What other reagent could be used for the same purpose?
Phosphoric acid (H₃PO₄)
Important info in SDS
Product Info, Hazards, Safe Handling, Physical & Chemical Properties
What is the role of sulfuric acid? What is its effect on reaction equilibrium and reaction rate?
Provide H+ for abstraction of water and formation of carbocation. Excess H+ shifts reaction to right
What is role of sodium bisulfite?
Quenches reaction by consuming excess oxidizing agent
Symmetrical vs. nonsymmetrical ether (with examples)
R-O-R vs. R-O-R* dibenzhydryl ether vs. benzhydryl-n-propyl ether
Benzoic acid + KOH rxn
RCOOH + KOH → RCOO⁻K⁺ + H2O
What is the equipment that returns solvent vapors to the reaction pot?
Reflux condenser
Synthesis of camphor: What is the purpose of the sodium bicarbonate wash?
Remove acidic impurities
Why is simple distillation performed?
Remove heptane solvent. Less time than fractional distillation
What is the purpose of adding magnesium sulfate to the organic layer? What other reagent could be used in its place?
Remove water/dry solution. Calcium chloride (CaCl₂), sodium sulfate (Na₂SO₄)
What factors contribute to the stability of the benzhydryl cation?
Resonance of two adjacent benzene rings
Which distillation technique, simple distillation (S) or fractional distillation (F) would be more suitable for the following operations? o Preparing drinking water from seawater? o Removing diethyl ether (BP 35°C) from para-dichlorobenzene (184°C)? o Separating benzene (BP 80°C) from toluene (BP 111°C)?
S, S, F
Water is a co-product of the reaction--how is it removed?
Separatory funnel + drying reagents
What's better/different about fractional distillation?
Slower, but better separation of similar BP liquids, uses fractional distillation column between flask and distillation head
What does (aq) mean?
Solution with water as the solvent
What to do with splash of strong acid?
Strip off gloves/lab coat, alert TA, wash affected area with water for 10-15 min
What does "neat" refer to when taking an IR spectrum?
The sample is prepared as a thin film, with no solvent present
What is the purpose of washing the organic layer with sodium bicarbonate?
To neutralize H+ present
What is the purpose of washing the organic layer with saturated sodium chloride?
To remove water
A separatory funnel contains water, diethyl ether, benzoic acid, naphthalene, and 2M sodium hydroxide. What are the layers?
Upper: TBME dissolving naphthalene Lower: water dissolving sodium benzoate and Na+ OH-
Safety issues of HCl and NaOH
Very caustic (skin & eyes)
Advantage of slowly grown large crystals over quickly grown small crystals
Will have better formed crystal lattice structure with fewer impurities
Would sulfuric acid be appropriate for the same purpose?
Yes, but need less since sulfuric acid has 2 acidic hydrogens
Rank the following solvents in order of polarity: diethyl ether, ethanol, acetic acid, toluene
acetic acid > ethanol > diethyl ether > toluene
Name a ketone encountered in this lab
acetone
Rank the following solvents in order of polarity: hexane, ethyl acetate, acetone, methanol
acetone > methanol > ethyl acetate > hexane
Drying agents: efficiency
amount of water left in the organic solution after drying
Name a carboxylic acid encountered in this lab
benzoic acid
Functional groups of benzoic acid and 1,4-dimethoxybenzene
benzoic acid (carboxylic acid, benzene ring) 1,4-dimethoxybenzene (2 esters)
Name an ether encountered in this lab
benzydryl-1-propyl-ether, dimethyl ether
Rank the following functional groups by Rf value: alcohol, aldehyde, carboxylic acid, aromatic ether
carboxylic acid < alcohol < aldehyde < aromatic ether
Stationary phase (column)
contains analyte/absorbent system, remains in column
Second crop
crystals recovered from mother liquor
Purpose of acetic acid in a solvent system?
decreases pH, suppresses ionization of any carboxylic acids present. Acidic compound can move along plate without streaking
Solute
dissolved substance in a solution
Name an alcohol encountered in this lab
ethanol
Main hazard of organic solvents
flammable
Safety issues of TBME
flammable, skin/eye irritant
eluent
fluid running out of tip of column
Features of good recrystallization solvent?
good solvent at high temperature, but not good solvent at lower temperatures; lowest mp of the ones that work
Name an alkane encountered in this lab
heptane
Drying agents: chemical inertness
how well the agent avoids reacting with the solvent and chemical
Rf changes with increasing solvent polarity
larger Rf values: solvent pulls polar compounds up, kicks nonpolar compounds up
Which compound has highest Rf and why? benzhydrol, 1-propanol, benzhydryl-n-propyl ether
least polar: benzhydryl-n-propyl ether. Ether makes O less polar
Solvent
liquid in excess that dissolves a substance
mother liquor
liquid left after initial crystallization (solute still present)
Distillate
liquid product condensed from vapor during distillation
Miscible
liquids able to be mixed together (oil and water would be immiscible)
Forerun
liquids whose BPs are lower than that wanted, usually impurities
Conjugate base, and example
matching base to acid in acid/base rxn (salicylic acid -> sodium salicylate)
Drying agents: capacity
maximum # of moles of water the drying agent can bind
Given that compound B can be recrystallized from methanol, ethanol and water, which would you choose and why?
methanol - lowest BP, easiest to dry
Why might you get a % recovery of salicylic acid that is greater than 100%?
might not be enough water to dissolve all of the NaCl created, so some undissolved NaCl was filtered out with the salicylic acid
TLC solvent system
mixture of developing solvents for TLC (i.e. hexane and EtAc)
Molecular symmetry and melt point
more symmetrical = higher melt point
Which would have a higher Rf value: naphthalene or beta-naphthol?
naphthalene: B-naphthol has an -OH group, making it more polar
NaOH spill on bench
neutralize with 5% acetic acid, cover with spill pad, dispose in solid waste
"Like dissolves like"
nonpolar dissolves nonpolar, polar dissolves polar
In hydrolysis of ester group, what species is the nucleophile and which is the electrophile?
nucleophile: -OH electrophile: O of ester
Experiment 1: two layers develop, what is in each?
organic (upper, TBME, 1,4-dimethoxybenzene), aqueous (lower, H2O, sodium benzoate, excess NaOH)
What compounds can be observed under UV light, after TLC analysis?
organic compounds containing pi bonds
Rank the following functional groups by Rf value: ketone, ester, phenol, alkene
phenol < ketone < ester < alkene
How is IR of product distinct from starting materials? What if it was contaminated?
product has ether peak and C-H (CH3) peak if contaminated, would see alcohol (C-O and O-H) peaks
Purpose of NaOH
provides separation of organic and aqueous layers, deprotonates benzoic acid into sodium benzoate so it dissolves in aqueous layer
Definition of melt point
range of temperatures, solid to liquid
What is the purpose of adding HCl to the cooled reaction mixture of disodium salicylate?
reacts with excess NaOH, neutralizing the solution Acidic solution allows disodium salicylate to be protonated and precipitate as salicylic acid
% recovery vs. % yield
recovery: recrystallization yield: synthesis reaction
What's the purpose of the NaCl wash of the organic layer?
remove majority of excess water in organic layer
Name a phenol encountered in this lab
salicylic acid
What would an explanation for if the mp of salicylic acid is higher than expected?
sampling height is too high (more heat needed)
Mobile phase (column)
solvent systems that passes through the stationary phase
Filtrate
the liquid left after filtration
temperature plateau
the temperature peak at which distillation is signaled to be over, is the BP of the lower boiling compound
What does [O] mean?
unspecified oxidizing agent
What makes a good TLC spotting solvent?
very volatile, nonreactive, should not absorb UV radiation (i.e. EtOAc)
What hot solvent will best dissolve benzoic acid: pentane or water? (think about salicylic acid)
water, more polar
boiling
when vapor pressure = atmospheric pressure
Too far below the branch in the distillation head?
will read as too high temperature, won't get good separation (compound with lower BP won't actually get distilled off)
If product was contaminated by benzhydrol, what would TLC plate look like?
would have another spot below the benzhydryl-n-propyl ether, more polar (alcohol)
Can KOH be used in place of NaOH to same end result?
yes, even stronger base so rxn still goes forward