chapter 10

Question 1

hydrocarbons tend to have?

Answer 1

• relatively low melting and boiling points because the only intermolecular forces they experience are London dispersion forces
  
  • the melting points of alkales and alkenes are primarily influenced by molecular weight (higher molecular weight → higher boiling/melting point and to a lesser extent branching (more branched → lower melting/boiling point because they stack less efficiently)
• have no acid-base chemistry to speak of, very stable and not particularly reactive

Question 2

haloalkanes or alkyl halides are alkanes with halogen subtituents (F, Cl, Br, or I) and can be made from?

Answer 2

• alkanes through a free radical mechanism
• from alkenes through hydrohalogenation
• or from alcohols through substituetion with hydrogen halides

Question 3

haloalkanes are quite reactive so they readily undergo?

Answer 3

• substitution and elimination reactions because halogens are generally excellent leaving groups (they are highly EN)
  *

Question 4

alkenes are important for the MCAT because?

Answer 4

• alkenes and alkene derivatives are the prototypical exmaples used to illustrate E/Z (cis/trans) isomerism
• molecules with alternating C=C double bonds form conjugated systems of bonds, which involve connected p-orbitals with delocalized electrons and these systems are often visualized well in UV spectropscopy
• alkenes provide a useful example for redox reactions
  
  • for ex. alkenes can be reduced in a process known as hydrogenation, in which an alkene reacts with H₂ in the presence of a catalyst (Ni, Pd, or Ot) to form an alkane
  • they can also be oxidized in a variety of ways
    
    • ex. using cold, dilute KMnO₄ to form a 1,2-diol

Question 5

phenol substituents:

Answer 5

• ortho- closest
• meta- middle
• para- paralel

Question 6

in general alcohols are?

Answer 6

• weakly acidic and their conjugate bases (alkoxides) are weak bases
  
  • pKa values in the range of 15-17, making them weaker acids than water (pKa of 14)
  • phenols are more acidic due to resonance stabilization of the conjugate base (pKa of 10)
  • the acidity of alcohols and phenols can be increased by adding EWG that help stabilize the negative charge on the conjugate base or decreased by adding electron-donating substituents

Question 7

the largest set of reactions involving alcohols is?

Answer 7

• oxidation
  
  • conversion of a hydroxyl group bound to a carbon to a carbonyl group or a carboxylic acid
    
    • first, oxidation to a carbonyl group and then further oxidation to a carboxylic acid and the outcome of the oxidation depends on whether the alcohol is primary or secondary amd on the oxidizing agent used
  • PCC is a weak oxidizing agent that turns C-OH into a C=O group and when applied to a primary alcohol, an aledehyde occurs whereas in secondary alcohols, a ketone results
  • stronger oxidizing agents oxidize primary alcohold to carboxylic acids and oxidize secondary alcohols into ketones
    
    • ex. hydroquinone to quinone through oxidation

Question 8

how do we protect OH groups from being oxidized or reduced?

Answer 8

• treating it with another compound that makes the -OH group non-reactive. once the desried reaction is completed, the protecting groups can be removed
  
  • three major classes are: silyl ethers (Si-O bond that can be broken with fluoride after the desired reaction is completed), myselates and tosylates (formed by reacting the alcohol with methylsulfonyl chloride and toluenesulfonyl chloride; they are good leaving groups so that can be used to protect or facilitate a substitution reaction in which the hydroxyl group is replaced by somthing else)
• acetals with acid workup

Question 9

the physical and chemical properties of aldehydyed and ketones are determined by the?

Answer 9

• carbonyl group
  
  • contains a single strong dipole meaning that aldehydes and ketones experiences dipole-dipole interactions so this raises their melting and boiling points compared to alkanes, but these interactions are not as strong as the H bonds that is characteristic of alcohols so the melting/boiling points of aldehydes and ketones are between those of alkanes and alcohols
  • in terms of acidity and basicity, the action for aldehydes and ketones takes place with the alpha-hydrogen and has resonance stabilization through an enolate structure (weakly acidic, less so than water)
  • involved in nucleophiles and electrophiles reactions
  • can be oxidized or reduced
    
    • aldehydes can be further oxidized to carboxylic acids by any oxidizing agent stronger than PCC. ketones cannot be further oxidized because it would be impossible to do so without breaking a C-C bond. aldehydes and ketones can be reduced to alcohols both by NaBH₄, a milder reducing agent, and LiAlH_4, a stronger reducing agent

Question 10

Nucleophilic addition reactions to the carbonyl carbon:

Answer 10

Nucleophile: What happens: Product: Notes:

1. water, water adds to the carbonyl C, deminol diol (two -OH groups on the cabronyl C), this process is reffered to as hydration
2. alcohol, ROH adds to the carbonyl C; in second round of addition the original carbonyl O is protonated and leaves. hemiacetal/hemiketal (one eq. of ROH) acetal/ketal (2 equivs of ROH), in lab conditions, reaction usually goes to completion and acetals/ketals are formed
3. Hydride reagents (NaBH₄ and LiAlH₄), hydrogen is added to the carbonyl group, alcohol, this is how aldehydes and ketones are reduced
4. amine (NH₃, etc.), the nitrogen from the amine replaces the carbonyl oxygen; a C=N double bond is formed, imine, imines undergo tautomerization and form an equilibrium with enamines
5. Hydrogen cyanide (HCN), the carbon in the deprotonated CN^- ion added to the carbonyl carbon, cyanohydrin (former carbonyl carbon now has an -OH group and a C triple bond N group)

Question 11

aldehydes and ketones can react with each other in an important reaction known as?

Answer 11

• aldol condensation
  
  • the basic mechanism involves the nucleophilic enolate ion of one aldehyde/ketone attacking the electrophilic carbonyl carbon of the other. the molecules join together and the immesiate result is an aldol (a molecule that combines the features of an aldehyde or ketone and an alcohol) and is technically known as beta-hydroxyaldehyde or beta-hydroxyketone. this simply refers to the fact that there is a single carbon separating the carbonyl and hydroxyl functional groups
  • the condensation occurs when the OH group is removied through an elimination reaction catalyzed by a strong base, resulting in an allpha,beta-unsaturated eldehyde or ketone or as a conjugated enone
    
    • the aldol condensation process is reversible and its called a retro-aldol reaction and it breaks the bonds between the alpha and beta carbons and is favoured by high temps and basic conditions and catalyzed in physiological conditions (4th step of glycolysis)

Question 12

carboxylic acids are characterized by a -COOH functional group while in carboxylic acid derivatives, the -OH group is?

Answer 12

• replaced by something else; that is all carboxylic acid derivatives essentially have a carbonyl group plus something (amides, esters, and anhydrides)

Question 13

carboxylic acids physical properties:

Answer 13

• ability to H bond
• quite acidic for organic weak acids
• the alpha-hydrogens on the carbon chain can be acidic in certain structures known as beta-dicarboxylic acids

Question 14

a special case of acid-base chemistry involving carboxylic acids is known as?

Answer 14

• saponification
  
  • under basic conditions, carboxylic acids are deprotonated and their conjugate bases form salts

Question 15

amides are?

Answer 15

• carboxylic acid derivatives with an amine (-NH₂, -NHR, NR₁R₂) attached to the carbon instead of the -OH gorup
  
  • cyclic acmides are known as lactams
    
    • beta-lactams form a very large class of broad-spectrum antibiotics, including penicillin and its derivaties

Question 16

esters are?

Answer 16

• the -OH group is replaced by an -OR group
  
  • cyclic esters are known as lactones
• do not engage in acid-base chemistry

Question 17

acid anhydrides are?

Answer 17

• formed by the condensation of 2 carboxylic acids

Question 18

a strong reducing agent is needed to reduce carboxylic acids to?

Answer 18

• alcohols
  
  • LiAlH₄ and DIBAL or DIBALH but only to include one unit of DIBAL for each unit of carboxyklic acid (NOT NaBH₄)

Question 19

carboxylic acids with another carbonyl group separated by a single carbon (1,3-dicarboxylic acids and beta-ketoacids) can also lose a -COOH group in a process known as?

Answer 19

• decarboxylation
  
  • in which the -COOH group is lost as CO₂ and this process is favoured by high temperatures and can be upregulated by enzymatic acitivity
  • occurs in the conversion of pyruvate to acetyl-CoA and at 2 points in the Krebs cycle

Question 20

carboxylic acids undergo many reactions involving?

Answer 20

• nuclephilic substitution at the carbonyl carbon
  
  • carboxylic acids have an excellent leaving group (OH)
    
    • amides are formed when ammonia or an amine acts as a nuclephile, esters are formed when an alcohol acts as a nuclephile in a process known as Fischer esterification and acid anhydrides are formed when a carboxylic acid acts as the nucleophile. these are considered condensationr eactions because in each of these reactions, 2 larger molecules are joined together with the loss of water

Question 21

what is Hell-Volhard-Zelinsky halogenation?

Answer 21

• carboxylic acids are halogenated at the alpha-carbon due to the slight acidity of the alpha-hydeogen
• using bromination as an example. this reaction occurs by the addition of a catalytic amount of PBr₃, followed by Br₂
  
  • first an acyl halide is formed in which a -Br substituent replaces the -OH. at this point, keto-enol tautomerism generates an enol form, and the Br₂ can react with the alkene bond of the enol to brominate the alpha-carbon. the first bromine is then spontaneously lost

Question 22

more reactive carboxylic acid derivatives can be?

Answer 22

• converted to less reactive carboxylic derivatives through nucleophilic substitution, but not vice versa

Question 23

what is transesterification?

Answer 23

• special reactions involving carboxylic acid derivatives
  
  • occurs when an ester is reacted with an alcohol distinct from the original alcohol that makes up the ester. the result is that these alcohol molecules replace each other, resulting in a different elcohol and a different ester

Question 24

another reaction involving carboxylic acids is the hydrolysis of amides to the parent carboxylic acid and an amine:

Answer 24

• the only interconversion reaction that amides undergo
• the hydrolysis of amides is the mechanism involved in breaking peptide bonds in amino acid sequences

Question 25

amines, imines and enamines are?

Answer 25

• nitrogen-containing compounds
  
  • amines are derivatives of ammonia (NH₃) in which one or more of the hydorgens ar ereplaced by a single bond to an organic substituent, prefix amine and suffix amino, weak bases, amines are defined by single bonds between nitrogen and carbon
  • imines are characterized by a C=N double bonds
  • enamines have a C=C bond and a C-N bond
    
    • imines and enamines are tuatomers that interconvery in a process very similar to keto-enol tautomerism

Question 26

3 sulfur-containing groups:

Answer 26

• thiols (-RSH)
• sulfides or thioesters (R-S-R’)
• thioesters (R-CO-S-R’) highly reactive
• disulfides (R-S-S-R’)