mcat o.chem
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- List the carboxylic acid derivatives in order of increasing reactivity.
- Amides < Esters < Acid Anhydrides < Acyl Halides
- How can amines be obtained from other nitrogen-containing functionalities (such as nitros or amides)?
- Amides can be made by reduction of other nitrogen-containing compounds: 1) From nitro compounds: - Use of Zn or Fe to reduce -NO2 groups to -NH2 - Especially useful for aromatic compounds (nitration of aromatic rings is easy) 2) From nitriles: - Reducing a -CN to produce primary amines - Uses LiAlH4 3) From iminies: - Adding H2 + Ni to an imine 4) From amides: - Adding LiAlH4 to an amide can form the amine
- Describe the reaction of esters with Grignard reagents.
- - Grignard reagents add to the C=O group to form ketones - However, these ketones are even more reactive, and are readily attacked by more Grignard reagent if available - Two equiv of Grignard can be used to produce 3* alcohols with GOOD YIELD
- Descrbie the Claisen condensation.
- - Enolate ion of one ester acts as a nucleophile, attacking another ester - Mechanism is analogous to that of the aldol condensation
- How can esters be reduced?
- - Reduction can occur by LiAlH4 but NOT by NaBH4 - Esters are reduced to promary alcohols
- Describe the basicity of amines.
- Amines are bases and readily accept protons to form ammonium ions - More basic than ammonia, but less basic than hydroxide
- Describe the hydrolysis of esters.
- - Hydrolysis yields carboxylic acids and alcohols - Can occur under acidic or basic conditions 1) Acidic conditions - C=O is protonated and H2O is nucleophile 2) Basic conditions - C=O is NOT protonated, and OH- is nucleophile
- What types of amines can be used to form amides, and why?
- Only primary and secondary amines can be used to form amides - This is because a loss of hydrogen is required
- What are two ways in which an amide can be converted into the corresponding amine?
- 1) Hoffman rearrangement = converts amides to primary amines with loss of the carbonyl 2) Reduction - LiAlH4(LAH) reduces amides to amines
- What are two ways in which acid anhydrides by synthesized?
- 1) Reaction of acid chloride with a carboxylate anion 2) Heating of carboxylic acids - Produces certain cyclic anhydrides
- What carboxylic acid derivatives can acid anhydrides be converted into, and how? (Use order of reactivity to help answer this)
- 1) Conversion to RCOOHs by hydrolysis 2) Conversion into amides by ammonia - NH3 cleaves anhydrides 3) Conversion to esters by reaction with alcohols **NOTE: All of these reactions produce a carboxylic acid as a side product
- Describe 3 ways in which carboxylic acids can be synthesized.
- 1) Oxidation - Of aldehydes, 1* alcohols, and alkylbenzenes - Usually by KMnO4 2) Carbonation of organometallic reagents - Gringnard + CO2 3) Hydrolysis of nitriles - Nitrile is a compound containing -CN - Can be hydrolyzed to form carboxylic acids and ammonia
- Name and describe the 3 types of "nucleophilic substitution" reactions that carboxylic acids can undergo.
- 1) Reduction - Attack by a H- nucleophile (LiAlH4) - Reduces all carboxylic acids to the alcohol 2) Ester formation - Occurs under acidic conditions (protonates the C=O) - Alcohol attacks the carbonyl, which is more electrophilic after protonating the O 3) Acyl halide formation - Performed by SOCl2
- Which carboxylic acid derivatives can acyl halides be converted into, and how? (Use the order of reactivity to help answer this)
- 1) Conversion to RCOOH by hydrolysis - React rapidly with water 2) Conversion to acid anhydrides by reaction with carboxylate anion 3) Conversion to esters by reaction with alcohol 4) Conversion to amides by reaction with amines
- Describe how the acidity of a carboxylic acid can be affected by additional substituents on the adjacent carbon atom.
- - Electron-withdrawing groups (-Cl or -NO2) INCREASE acidity - Electron-donating groups (-NH2 or -OCH3)DECREASE acidity
- Describe the acidity of the hydrogens in dicarboxylic acids.
- - One -COOH group influences the other, making the first H to be taken off more acidic - However, once the first H is removed, the newly formed carboxylate anion DECREASES the acidity of the other H on the second -COOH group - Therefore, the second H of a dicarboxylic acid is LESS acidic than that of a monocarboxylic acid
- Describe the formation of acetals and ketals from aldehydes and ketones.
- Aldehyde: - Hemiacetal = formed by addition of one equivalent of an alcohol - Acetal = formed by addition of another equivalent of alcohol; C is bonded to OR, OR, and H Ketone: - Hemiketal = formed by addition of 1 equiv of alcohol - Ketal = formed by addition of another equiv of alcohol; C bonded to OR, OR and no Hs
- Define/describe structural isomers.
- - Compounds that share ONLY molecular formula - Very different chemical and physical properties
- Define/describe geometric isomers.
- - Compounds that differ in position of substituents attached to a double bond. - Can be labeled "cis" and "trans" or "E" and "Z" - Differ in ALL physical properties, and in many chemical properties.
- How are enantiomers different from or similar to one another?
- - They have identical physical properties - Have most of the same chemical properties ** Rotate plane-polarized light in opposite directions (+ or -)
- What is the specific rotation of a compound?
- Specific rotation = [A] Observed rotation = a [A] = a/(concentration*length)
- How are diastereomers different from or similar to one another?
- - May have different physical properties (such as solubility) -- Can be separated by physical means
- Define/describe conformational isomers.
- - Differ only by rotation about one or more single bonds - These isomers are the most similar to one another out of all of the types of isomers
- What are the 3 types of ring strain that arise in cycloalkanes?
- 1) Angle strain -- bond angle deviations 2) Torsional strain -- eclipsed conformations of cyclic molecules 3) Nonbonded strain -- atoms or groups compete for the same space (van der Waals repulsion) **All of these are alleviated by adopting nonplanar conformations (chair, boat, envelope, etc)
- What general types of compounds take part in nucleophilic substitution reactions?
- Alkyl halides Other substituted carbon atoms
- Descrbie the trend of nucleophilicity for different compounds (same attacking atom, different attacking atom, protic/aprotic solvents, etc).
- - If the attacking atom is the same, nucleophilicity increases as basicity increases. If the attacking atoms differ: - In protic solvents, nucleophilicity correlates with size because large atoms can shed their solvent molecules better - In aprotic solvents, nucleophilic strength correaltes with basicity (because they are "naked" -- not solvated)
- Describe what characterizes a good leaving group.
- **Note -- the ease with which nucleophilic substitution takes place is dependent on the LG as well as the nucelophile Best leaving groups are WEAK BASES (can dissociate to form a stable species)
- What factors affect the rate of SN1 reactions (what favors SN1 reactions)?
- For SN1 --> rate-determining step is formation of carbocation - Rate of SN1 DEPENDS ONLY ON THIS ONE MOLECULE - The Rate can NOT be affected by the strength of the nucleophile Rate of SN1 reaction can be increased by: - Highly substituted carbon atoms (stable carbocations) - Polar, protic solvents --> stabilizes the carbocation by solvation - Better LGs (original substituent should be better LG than nucleophile)
- What factors determine the rate of SN2 reactions (what favors SN2 reactions)?
- Since the rate-determining step involves two species (the substrate and the nucleophile), the rate can be affected by both of these: - Nucleophile must be strong - Reactant cannot be sterically hindered (less substituted carbons) - Favored in APROTIC solvents
- Describe the physical properties of alkenes (in comparison to the corresponding alkanes).
- - Melting points and boiling points increase with molecular weight - Both are similar in value to those of the corresponding alkanes Trans alkenes have higher MELTING points because of higher symmetry and no net dipole Cis alkenes have higher BOILING points because the net dipole adds to the intermolecular forces
- What factors favor E1 reactions? How does this compare to SN1 reactions? How can you direct a molecule to one reaction over the other?
- E1 reactions are favored by the same factors that favor SN1: - Highly polar, protic solvents - Highly substituted carbon chains - Good LGs - Weak nucleophiles in low concentration E1 and SN1 are very competitive --> hard to direct a reaction toward one over the other ** High temperatures favor E1! **
- What factors favor E2 reactions? Also, what is REQUIRED in order for an E2 reaction to even be possible?
- - Steric hindrance does not affect E2 (unlike SN2) - Strong, bulky bases favor E2 **The base will remove a proton that is ANTI to the LG - The two things that are being removed must be anti to one another
- Compare E2 to SN2. How can you direct a reaction towards one over the other?
- 1) E2 reactions are not affected by steric hindrance, but SN2 reactions require little steric hindrance. Therefore, E2 reactions can proceed with highly substituted carbon chains. 2) A strong base favors E2 over SN2. SN2 is favored by weak Lewis bases (strong nucleophiles).
- What is the process of catalytic hydrogenation?
- - Reductive process of adding H2 to a double bond, with the aid of a metal catalyst (Pd, Pt, or Ni) - Addition is "syn" --> both protons are added to the same side
- Describe the process of hydroboration for alkenes and alkynes.
- - Addition of B2H6 to double and triple bonds For alkenes: - Addition is anti-Markovnikov - Produces an alcohol with SYN orientation For alkynes: - Addition is anti-Markovnikov - Produces the aklene with CIS orientation
- What does the addition of KMnO4 do to alkenes (cold, dilute vs. a basic solution)?
- Cold, dilute KMnO4: - Produces diols with SYN orientation Basic KMnO4 (KMnO4, OH-, heat): - Cleaves the alkene to form two carboxylic acids
- Describe the physical properties of alkynes (as compared to the analogous alkenes and alkanes).
- - Similar to those of the analogous aklenes and alkanes - Internal alkynes boil at HIGHER temps than terminal alkynes (more stable)
- What are two ways to accomplish partial hydrogenation of alkynes (hydrogenation stopping at the aklene)?
- 1) Lindlar's catalyst - Using H2, Pd/BaSO4, quinoline - Produces the CIS alkene, because reaction occurs on the metal's surface 2) Na, NH3 (liquid) - This produces the TRANS isomer of the alkene
- What do KMnO4 and ozone do to alkynes?
- 1) KMnO4, OH- (basic potassium permanganate) - Cleaves the triple bond and forms two carboxylic acids 2) Ozone - Also cleaves the triple bond to form carboxylic acids
- What conditions are required for a molecule to be considered aromatic?
- 1) Must be cyclic 2) Must be conjugated (alternating single and double bonds) 3) Must follow Huckel's rule --> 4n + 2 pi electrons
- Describe the properties of aromatic compounds (and compare them to other hydrocarbons).
- Generally same physical properties as other hydrocarbons Chemical properties are significantly different: - Very stable - Very unreactive
- Describe the four types of electrophilic aromatic substitutions.
- 1) Halogenation - Adding FeCl3, FeBr3, AlCl3, etc. 2) Sulfonation - Adding HSO3 - Uses SO3/H2SO4 as reactants 3) Nitration - Adding NO2 - Uses HNO3 as a reactant 4) Friedel-Crafts Acylation - Adding a carbonyl compound - Catalyzed by a Lewis acid, such as AlCl3
- Describe the catalytic reduction of aromatic rings.
- - Benzene rings can be reduced by catalytic hydrogenation - Using H2, Rh/C - Under HIGH temperatures - Yields a cyclohexane ring
- Describe the acidity of the H on the OH of an alcohol. Of a phenol? How do ring substituents affect the acidity of the phenol H?
- The hydroxyl H is weakly acidic The hydroxyl H of phenols are more acidic than those of alcohols - This is due to resonance structures Ring substituents: - Electron-withdrawing substituents INCREASE acidity - Electron-donating groups DECREASE acidity
- What are the 3 main ways by which an alcohol can be synthesized?
- 1) Addition reactions - Addition of water to a double bond - Addition of organometallic compounds to carbonyl groups 2) Substitution reactions - SN1 or SN2 can be used 3) Reduction reactions - Reduction of aldehydes, ketones, carboxylic acids, or esters - Often done using LAH or NaBH4
- Why are substitution reactions with alcohols limited? What are 3 ways in which this limitation can be removed?
- Displacement of OH groups in substitutions is rare --> OH ion is a poor leaving group Must be made into a good LG for substitution to occur: 1) Protonate it 2) Convert it to a tosylate 3) Form an inorganic ester - Treating with SOCl2 or PBr3 to convert an alcohol into an alkyl halide
- Name 3 reagents that perform oxidation of alcohols (and what products are formed).
- 1) PCC = mild oxidant - Converts 1* alcohols into aldehydes 2) KMnO4 = strong oxidant - Oxidizes alcohols all the way to carboxylic acids - Forms ketones from 2* alcohols 3) CrO3, H2SO4, acetone (Jones' reagent) = strong oxidant - Same as KMnO4 **Note -- oxidation of alcohols generally involves chromium
- Describe the physical properties of ethers (compared to alcohols and corresponding alkanes).
- - Do not undergo H-bonding - Boil at relatively low temperatures compared to alcohols - Boil at about same temps as corresponding alkanes. - Only slightly polar --> only slightly soluble in water
- How are standard ethers synthesized? What is the specific reaction mechanism?
- Williamson Ether Synthesis: - Ethers produced by reaction of metal alkoxides with PRIMARY alkyl halides - Proceeds via SN2 reaction **NOTE: Alkoxides will attack ONLY nonhindered halides (SN2)
- Describe the process of cleaving ethers.
- Cleavage of ethers only occurs under vigorous conditions: - High temperatures - In presence of Hbr or HI **NOTE: The aloxide of the ether must be protonated before it leaves, because otherwise it would be a strong base, and strong bases are BAD LGs! After protonation --> cleavage proceeds by either SN1 or SN2, depending on the structure of the ether.