chem 263 lab exam
What does each signal in a ¹H NMR correspond to?
A group of chemically equivalent protons.
What is the reference compound used in NMR and where does it appear?
Tetramethylsilane (TMS); appears at 0.0 ppm as a single sharp peak.
What does downfield mean?
Higher ppm value (left side of spectrum); protons are more deshielded.
What does the integral of a peak measure?
The area under the peak, proportional to the number of protons giving that signal.
Do integration values give absolute or relative numbers of hydrogens?
Relative numbers; they must be scaled to match the total number of H in the molecule.
What causes splitting (multiplicity) in ¹H NMR?
Coupling with non-equivalent neighboring protons 2–3 bonds away.
Why do OH and NH protons often appear as broad singlets?
Because of rapid proton exchange, causing spin decoupling (no clear coupling pattern)
What is a coupling constant (J)?
The distance between lines in a multiplet, measured in Hz.
What is typical J for vicinal (3-bond) coupling?
About 6–8 Hz.
How does vicinal coupling compare to long-range (4-bond) coupling?
Vicinal coupling is stronger, long-range is weaker/smaller J.
What is meant by reciprocity of coupling constants?
If proton A couples to proton B, they share the same J value between them.
What does a doublet of doublets (dd) indicate?
The proton is being split by two different sets of non-equivalent neighbors, each with its own J.
What does a doublet of triplets (dt) indicate?
Splitting by one set (n₁) giving a doublet and another set (n₂) giving a triplet → combined dt.
What does a doublet of quartets (dq) indicate?
One coupling to one proton (doublet) and another coupling to three protons (quartet).
What does ddd (doublet of doublet of doublets) suggest?
The proton couples to three different sets of non-equivalent protons
When are protons chemically equivalent?
When they can be interchanged by rotation or a symmetry operation (axis, plane).
How does symmetry affect the number of NMR signals?
Symmetry makes some protons equivalent, so you see fewer signals than the number of H atoms.
What is the purpose of the Diels–Alder experiment?
To perform a [4+2] cycloaddition between α-terpinene (diene) and an N-phenylmaleimide derivative (dienophile) to form a bicyclic cycloadduct.
What is green chemistry?
An approach that aims to make chemical processes safer and more environmentally friendly by reducing waste, using safer solvents, improving atom economy, and minimizing hazards.
Which green chemistry principle is demonstrated by this experiment’s solvent choice?
Use of benign solvents — CPME is a greener, safer alternative to traditional organic solvents.
Why is CPME used as the solvent in this reaction?
It is a green solvent, low-toxicity, stable, and has a suitable boiling point for refluxing the reaction.
What role does α-terpinene play in the reaction?
It is the diene required for the [4+2] cycloaddition.
What role does the N-phenylmaleimide derivative play in the reaction?
It is the dienophile, an electron-poor alkene that readily undergoes Diels–Alder cycloaddition.
Why is a slight excess of α-terpinene used?
To ensure complete consumption of the limiting reagent, the N-phenylmaleimide derivative.
What is the purpose of heating the reaction under reflux?
To maintain a constant elevated temperature so the reaction proceeds efficiently without losing solvent.
What is the purpose of adding methanol after the reaction?
Methanol precipitates the product, allowing it to crystallize by cooling.
Why is the methanol wash performed with ice-cold methanol?
Cold MeOH improves the purity of the solid by washing away impurities without redissolving the product.
What major change is expected in the IR spectrum after the Diels–Alder reaction?
Disappearance of alkene C=C stretches from both reagents and retention of the imide C=O peaks.
What information does the NMR spectrum provide in this experiment?
Confirmation of the cycloaddition product’s structure and identification of the R-group on the N-phenylmaleimide.
Which reagent is the limiting reagent in theoretical yield calculations?
The N-phenylmaleimide derivative.
Why is the Diels–Alder reaction considered “green”?
It has good atom economy, requires no strong reagents, produces minimal waste, and uses a safer solvent.
What type of product is formed in this Diels–Alder reaction?
A bridged bicyclic cycloadduct.
Why is cooling in an ice bath necessary before filtration?
To maximize crystal formation and product isolation.
What fundamental feature defines a Diels–Alder reaction mechanism?
It is a concerted reaction where two π bonds break and two σ bonds form simultaneously.
What type of reaction is a Friedel–Crafts reaction?
An electrophilic aromatic substitution reaction.
What are the two types of Friedel–Crafts reactions?
Alkylation and Acylation.
What is the role of FeCl₃ or AlCl₃ in Friedel–Crafts reactions?
They act as Lewis acid catalysts that generate the electrophile.
Why is FeCl₃ often used instead of AlCl₃?
It is easier to handle/storage, though both are hygroscopic.
In Friedel–Crafts alkylation, how is the electrophile (carbocation) formed?
By the Lewis acid forming a complex with the alkyl halide, which then dissociates into a carbocation and FeCl₄⁻.
What is the major drawback of Friedel–Crafts alkylation?
It can produce poly-alkylated products due to increased ring activation after the first substitution.
Why can rearrangements occur during Friedel–Crafts alkylation?
Because carbocations can rearrange to form more stable carbocations before reacting.
In the example reaction with 1-chloropropane, why is isopropylbenzene the major product?
A hydride shift creates a more stable secondary carbocation, which reacts preferentially.
What is the electrophile in Friedel–Crafts acylation?
An acylium ion (R–C≡O⁺).
Why do Friedel–Crafts acylations not undergo rearrangements?
The acylium ion is resonance-stabilized, so rearrangement is unnecessary/unfavorable.
What is the main organic product of Friedel–Crafts acylation?
An aryl ketone.
Why is anisole reactive toward electrophilic substitution?
t he methoxy group is electron-donating, activating the ring.
Where does substitution occur on anisole in this acylation?
Mainly the para position (less steric hindrance).
Why is a gas trap used in this reaction?
To safely vent HCl gas generated during the reaction.
What solvent is used in the Friedel–Crafts acylation experiment?
Dichloromethane (DCM).
When extracting, why must you avoid shaking too vigorously?
It can cause emulsions, which make layer separation difficult.
What can be added to break an emulsion?
Brine (saturated NaCl).
What is the purpose of adding distilled water during the workup?
t o quench the reaction, dissolve inorganic byproducts, and separate the layers.
What is the purpose of sodium sulfate in this experiment?
To dry the organic layer by removing dissolved water.
What analytical techniques are used to identify the product and unknown acyl chloride?
TLC, IR, and ¹H NMR.
What indicates a successful Friedel–Crafts acylation on IR?
Appearance of a strong C=O stretch (~1680–1750 cm⁻¹).
What byproduct gas is generated during Friedel–Crafts reactions?
Hydrogen chloride (HCl) gas.
In Friedel–Crafts acylation, which reagent is usually the limiting reagent?
The acyl chloride.
Why is the methoxy group para-directing in anisole?
Because lone-pair resonance donation increases electron density at the ortho/para positions.
What type of reagent is a Grignard reagent?
An organomagnesium halide (R–Mg–X).
What is the main purpose of a Grignard reaction?
To form carbon–carbon bonds by reacting the Grignard reagent with electrophiles.
Why must Grignard reactions be performed under anhydrous conditions?
Because Grignard reagents react with water or any acidic protons, destroying the reagent.
What solvents are typically used for Grignard reactions?
Anhydrous diethyl ether or THF.
What happens if a Grignard reagent contacts protic solvents?
It is protonated and forms an alkane, destroying the reagent.
What impurity is commonly formed during Grignard reagent preparation?
The alkyl dimer (R–R) from Wurtz coupling with unreacted alkyl halide.
What happens if Grignard reagents react with atmospheric oxygen?
Oxygen inserts into the Mg–C bond, forming hydroperoxides.
What do Grignard reagents form when reacting with aldehydes?
Secondary alcohols (except formaldehyde → primary alcohol).
What do Grignard reagents form when reacting with ketones?
Tertiary alcohols.
What do Grignard reagents form when reacting with CO₂?
Carboxylic acids (after acid workup).
What do Grignard reagents form when reacting with esters or acyl chlorides?
Tertiary alcohols after two additions of the Grignard reagent.
What is the electrophilic center attacked by a Grignard reagent?
The carbonyl carbon of aldehydes, ketones, esters, or acyl chlorides.
What is formed immediately after the Grignard reagent attacks a carbonyl?
An alkoxide intermediate.
What reagent is added after a Grignard reaction to protonate the product?
An acidic workup such as HCl or NH₄Cl solution.
What compound was used as the electrophile in this experiment?
4,4′-dimethoxybenzophenone (a ketone).
: What type of product is expected when a Grignard reagent attacks 4,4′-dimethoxybenzophenone?
A tertiary alcohol.
Why must glassware be flame-dried before a Grignard reaction?
To remove all traces of moisture, which destroy the reagent.
What is the function of the addition funnel in this experiment?
To add the Grignard reagent slowly (dropwise) to control the reaction.
Why is a calcium chloride drying tube used in the setup?
To prevent atmospheric moisture from entering the reaction.
What is the purpose of heating the reaction at reflux?
To ensure the reaction proceeds efficiently at a constant temperature.
What solution is used to quench the Grignard reaction?
Saturated aqueous ammonium chloride.
Why is NH₄Cl used for quenching?
It protonates the alkoxide and safely destroys excess Grignard reagent.
What organic solvent is used for extraction in this experiment?
Diethyl ether.
What major IR change confirms the reaction succeeded?
Loss of the C=O ketone stretch and appearance of O–H alcohol stretch.
What is the limiting reagent in this experiment?
The Grignard reagent (added in measured 1 equivalent).
What does “reduction” mean in organic chemistry?
Addition of hydrogen or removal of oxygen from a molecule.
What type of reagent is NaBH₄?
A hydride donor, delivering H⁻ to carbonyl groups.
What functional groups can NaBH₄ reduce?
Only aldehydes and ketones (at low temperatures).
What functional groups can LiAlH₄ reduce?
Aldehydes, ketones, esters, carboxylic acids, and amides.
Which reagent is more reactive: NaBH₄ or LiAlH₄?
LiAlH₄ is much more reactive.
Why must NaBH₄ reductions be performed in alcohol or aprotic solvents?
Hydride is a strong nucleophile and reacts violently with water.
What is the key mechanistic step in NaBH₄ reduction?
Hydride transfer to the carbonyl carbon.
What immediate intermediate is formed after hydride attack?
An alkoxide.
What is required after the alkoxide is formed?
An acidic workup to protonate the alkoxide → alcohol.
What does NaBH₄ reduce an aldehyde to?
A primary alcohol.
What does NaBH₄ reduce a ketone to?
A secondary alcohol.
What was the goal of the experiment?
Reduce an unknown aldehyde or ketone to identify which alcohol was formed.
Why is ethanol used as the solvent for NaBH₄?
It allows controlled hydride transfer while not instantly destroying NaBH₄.
Why must 5% HCl be added slowly during workup?
To quench NaBH₄ safely and avoid excessive gas release.=
What is the function of ethyl acetate during workup?
It forms the organic layer containing the alcohol product.
Why is brine used in the extraction?
To help remove dissolved water from the organic layer.
What TLC eluent is used in this experiment? nabh4 reduction
2:1 hexane : ethyl acetate.