9,10,11,12,13 Flashcards
Acetylene
“Acet-“: This prefix is related to the two-carbon unit found in the molecule. It stems from the Latin word “acetum,” meaning “vinegar.” This connection arises because acetic acid (the main component of vinegar) can be produced from acetylene through a series of chemical reactions.
“-ylene”: This suffix is the standard ending used to denote alkynes in chemical nomenclature. It signifies the presence of the carbon-carbon triple bond, which is the defining characteristic of this class of compounds.
Acetylide ion
- “ide” indicates the presence of a negative charge
Alkynide ion
True or False:
The conjugate base of a terminal alkyne, called an alkynide ion, can only be formed with a sufficiently
strong base.
True
Strong bases that can deprotonate acetylene or terminal alkyne
________ dihalide, which means that both halogens are con-
nected to the same carbon atom.
geminal
________ dihalide, which the two halogens are connected to adjacent carbon atoms:
vicinal
A poisoned catalyst will catalyze the conversion of an alkyne into a _______ alkene, but it will not catalyze the subsequent reduction to form the alkane
cis
Examples of _______________________
- Lindlar’s catalyst, which is a Pd catalyst prepared with CaCO3 and traces of PbO2.
- P-2 catalyst, which is nickel- boron complex (Ni2B).
poisoned catalysts
Alkynes can also be reduced to trans alkenes via a reaction called _________________________
dissolving metal reduction
To produce a _________ alkene, an Alkyne can be treated with sodium metal in liquid ammonia.
trans
When the starting alkyne is treated with excess H-X, two successive addition reactions occur, producing a ______________
geminal dihalide
In the presence of peroxides, HBr undergoes an _______________ addition across an alkene or terminal alkyne
anti-Markovnikov
*peroxides causes radical addition mechanism
Radical addition only occurs with HBr (not with HCl or HI)
In the context of alkynes, hydration (addition of OH & H) results in the formation of an______, which then tautomerizes to a ketone or aldehyde
enol
Hydration of Alkynes
Step 1: Addition of Water: Water adds across the triple bond in the presence of a strong acid catalyst (usually sulfuric acid, H2SO4) and mercury(II) sulfate (HgSO4).
Step 2: Tautomerization: The enol intermediate rapidly converts to a more stable ketone or aldehyde through a process called tautomerization.
*The regioselectivity of the reaction is governed by Markovnikov’s rule
Hydration of alkynes is a valuable synthetic tool for preparing ketones and aldehydes
Hydration of internal alkynes generally yields _________
ketones
Hydration of terminal alkynes produces ____________
aldehydes
The initial product of alkyne hydration is an enol, which quickly_____________ to the more stable carbonyl compound (ketone or aldehyde).
tautomerizes
H on the O relocates
The enol and ketone are said to be ____________, which are constitutional isomers that rapidly interconvert via the migration of a H
tautomers
- tautomers are constitutional isomers that exist in equilibrium with one another. Once the equilibrium has been reached, the concentrations of ketone and enol can be measured. In contrast, resonance structures are not different compounds and they are not in equilibrium with one another.
interconversion between an enol and a ketone is called _______________
keto-enol tautomerization
True or False:
Tautomerization cannot be prevented, and it is catalyzed by either acid or base.
True
An alkyne possesses 2 pi bonds. As a result, two molecules of BH3 can add across the alkyne. To prevent the second addition, a
dialkyl borane (R2BH) is employed instead of BH3. The two alkyl groups provide______________ that prevents the second addition.
2 commonly used dialkyl boranes are disiamylborane and 9-BBN
steric hindrance
Hydroboration- Oxidation of Alkynes, the initial product of this reaction is an enol that cannot be isolated because it is rapidly converted into an____________ via tautomerization.
aldehyde
Halogenation of Alkynes, excess or 2 equivalents of Br2 or Cl2 will add to form a ____________
tetrahalide
Halogenation of Alkynes, 1 equivalent of Br2 or Cl2 will add to form a ____________
dihalide
- E isomer (opposite sides due to anti addition)
When treated with ozone followed by water, Alkynes undergo oxidative-cleavage to produce ____________
carboxylic acids
When a terminal Alkyne undergoes oxidative-cleavage, the terminal carbon atom is converted into __________
carbon dioxide
What type of reaction does the acetylide ion undergo with a primary alkyl halide?
SN2 rxn
What type of reaction does the acetylide ion undergo with a secondary or tertiary alkyl halide?
E2
A terminal alkyne can be deprotonated by a strong base, such as sodium amide (NaNH2), the resulting alkynide ion can function as a nucleophile when treated with an alkyl halide and this transformation proceeds via an SN2 pathway and provides a method to install an alkyl group. This process is called __________ of Terminal Alkynes
Alkylation of terminal alkynes
*This process is only efficient with methyl or primary alkyl halides (SN2)
Acetylene possesses two terminal protons (one on either side) and can therefore undergo alkylation _________
twice
What is the stability trend for alkyl radicals?
Tertiary > Secondary > Primary > Methyl
What are some common reactions involving radicals?
Halogenation, addition reactions, and polymerization.
Are radicals electrophilic or nucleophilic?
Electrophilic due to their electron deficiency.
Is a carbon radical trigonal planar, trigonal pyramidal, or in-between the two?
A carbon radical is between these two cases, because it has one nonbonding electron. It therefore might seem reasonable to expect that the geometry of a carbon radical would be somewhere in between trigonal planar and trigonal pyramidal. Experiments suggest that carbon radicals either are trigonal planar or exhibit a very shallow pyramid (nearly planar) with a very low barrier to inversion
TREATED AS TRIGONAL PLANAR
What factors contribute to radical stability?
Hyperconjugation and inductive effects.
Order of stability of radicals?
Tertiary vs Allylic vs Benzylic
Benzylic > Allylic > Tertiary
There are six different kinds of arrow-pushing patterns that comprise radical reactions.
1.
2.
3.
4.
5.
6.
Homolytic cleavage
Addition to a π bond
Hydrogen abstraction
Halogen abstraction
Elimination
Coupling
__________ cleavage requires a large input of energy. This energy can be
supplied in the form of heat (Δ) or light (hν).
Homolytic cleavage
*initiation step in radical reaction
A radical adds to a π bond, thereby destroying the π bond and generating a new radical.
Addition to a pi bond
*a propagation step in radical reaction
A radical can abstract a hydrogen atom from a compound, generating a new radical. Don’t confuse this step with a proton transfer, which is an ionic step. In a proton transfer, only the nucleus of the hydrogen atom (a proton, H+) is being transferred. Here, the entire hydrogen atom (proton and electron, H*) is being transferred from one location to another.
Hydrogen abstraction
*a propagation step in radical reaction
A radical can abstract a halogen atom, generating a new radical. This step is similar to hydrogen abstraction, only a halogen atom is being abstracted instead of a hydrogen atom.
Halogen abstraction
*a propagation step in radical reaction
The position bearing the unpaired electron is called the alpha position. In an elimination step, a double bond forms between the alpha (α) and beta (β) positions. As a result, a single bond at the β position is cleaved, causing the compound to fragment into two pieces.
Elimination
*a propagation step in radical reaction
Two radicals join together and form a bond.
Coupling
*termination step in radical reaction
The step when radicals are created
Initiation step
The step when two radicals annihilate each other by forming a bond.
Termination step
2 important notes in the Propagation step of a radical reaction
- Formation of the products
- Regeneration of the radical
The first propagation step is a ____________ abstraction, and the second propagation
step is a ______________ abstraction.
hydrogen
halogen
In radical reactions, the sum of the ____________ steps gives the net chemical reaction.
propagation
Radical reaction (chlorination)
In order to produce monohalogenation, there needs to be a low concentration of the _____________
halogen
- excess halogen leads to polyhalogenation
Photochemical initiation requires the use of UV light, while thermal initiation requires a very high temperature (several hundred degrees Celsius) in order to produce the energy necessary to cause homolytic bond cleavage of a Cl-Cl bond. Achieving thermal initiation at more ‘moderate temperatures’ requires the use of a _______________, a compound with a weak bond that undergoes homolytic bond cleavage with greater ease.
radical initiator
- Alkyl peroxides
- Acyl peroxides
A_______________ is a compound that prevents a chain process from either getting started or continuing and they effectively destroy radicals
radical inhibitor (radical scavenger)
- O2
- hydroquinone
What is the trend of halogen reactivity in radical halogenation reactions?
Fluorine > Chlorine > Bromine > Iodine
What is the rate-determining step in a radical reaction?
The hydrogen abstraction sep of propagation
Radical reaction:
Why does bromination show greater ‘selectivity’ than chlorination?
The transition state for bromination is closer to the product (radical) in energy, making radical stability a more important factor in determining the reaction rate.
What is the relative reactivity of chlorine and bromine towards different types of hydrogen atoms?
Chlorine is less selective than bromine. It shows a smaller difference in reactivity between primary, secondary, and tertiary hydrogens.
What is the stereochemical outcome of radical halogenation at a chiral center?
Radical halogenation at a chiral center leads to a racemic mixture of products.
This is because the intermediate radical is planar, and the halogen can attack from either face with equal probability.
If there is an existing chiral center that is not involved in the radical halogenation
process. In such a case, the products are _______________, rather than enantiomers
diastereomers
What is radical allylic bromination of an alkene?
The substitution of a hydrogen atom at the allylic position with a bromine atom using N-bromosuccinimide (NBS).
- be careful high concentrations of Br2 will add across double bond instead
What is the one serious flaw with radical allylic bromination of an alkene?
When the reaction is performed with Br2 as the reagent, a competition occurs between allylic bromination and ionic addition of bromine across the π bond
so to avoid this competing reaction, the concentration of bromine (Br2) must be kept as low as possible throughout the reaction. This can be accomplished by using N-bromosuccinimide (NBS) as a reagent, instead of Br2
_____________ can rise into the stratosphere, where they are broken down by UV light, releasing chlorine radicals. These chlorine radicals can catalyze the destruction of ozone, disrupting the Chapman cycle and leading to ozone depletion.
CFCs
- The Montreal Protocol is an international treaty designed to phase out the production and use of ozone-depleting substances, including CFCs
What are some alternatives to CFCs?
Some alternatives to CFCs include hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluoro-olefins (HFOs).
- UV light can’t homolytically cleave a C-F bond
Ultimately, the ozone cycle in the stratosphere turns harmful UV radiation into __________, another form of energy
Heat
In the presence of atmospheric oxygen, organic compounds are known to undergo a slow oxidation process called ___________________
autooxidation
Naturally occurring fats and oils, such as vegetable oil, are generally mixtures of compounds called triglycerides, which contain three long hydrocarbon chains. The hydrocarbon chains often contain double bonds, which render them susceptible to_________________, specifically at the allylic positions where hydrogen abstraction can occur more readily. The resulting hydroperoxides contribute to the rancid smell that develops over time in foods containing unsaturated oils. Moreover, hydroperoxides are also toxic. Food products with unsaturated oils therefore have a short shelf life unless radical inhibitors are used to slow the autooxidation process. Many radical inhibitors are used as food preservatives, including BHT and BHA (antioxidants). Naturally occurring antioxidants are vitamin C & vitamin E.
autooxidation
What is the key difference between radical addition of HBr and regular HBr addition to alkenes?
Radical addition follows Anti-Markovnikov regioselectivity, while regular addition follows Markovnikov’s rule.
What conditions are necessary for radical addition of HBr?
Presence of peroxides (like ROOR) or other radical initiators.
What are some common polymers produced by radical polymerization?
Polyethylene, polystyrene, PVC, and Teflon.
How does the polymer chain grow?
Through the continuous addition of monomer units to the active radical chain end.
Most synthesis problems will have numerous correct answers,
although_______________ should always be a top priority (fewer
steps are preferable).
efficiency
What is the IUPAC nomenclature for alcohols?
Replace the -e ending of the parent alkane with -ol (e.g., methane becomes methanol).
How are the common names of alcohols formed?
Name the alkyl group followed by the word “alcohol” (e.g., methyl alcohol).
What are the products when an alcohol acts as an acid?
An alkoxide ion (RO-) and a proton (H+)
How does alkyl substitution influence alcohol acidity?
Increased alkyl substitution decreases acidity. Tertiary alcohols are less acidic than secondary, which are less acidic than primary.
What is the functional group of a phenol?
A hydroxyl group (-OH) connected to a benzene ring.
Compare the acidity of phenols and alcohols.
Phenols (pKa around 10) are significantly more acidic than alcohols.
- the phenoxide ion (conjugate base) is stabilized by resonance delocalization of the negative charge into the benzene ring.
Alcohols can be prepared by substitution reactions in which a leaving group
is replaced by a hydroxyl group. A primary substrate will require _______ conditions (a strong nucleophile), while a tertiary substrate will require _______ conditions (a weak nucleophile).
SN2
SN1
- For a secondary substrate, neither SN2 or SN1 is effective for preparing a secondary
alcohol (under SN1 conditions, the reaction is generally too slow & SN2 conditions will generally favor elimination over substitution)
Alcohols can be prepared by addition reactions via ____________, ____________, & ____________
Acid-Catalyzed Hydration (markovnikov but possible C+ rearrange)
Oxymercuration-demercuration (markovnikov & anti-addition)
Hydroboration-oxidation (anti-markovnikov & syn-addition)
Assigning Oxidation States:
We treat all bonds as if they were purely ionic, and we break them heterolytically, giving each pair of electrons to the more _________________ atom in each case.
electronegative
What are the two main functional groups that can be reduced to form alcohols?
Aldehydes and ketones
What are the two common reducing agents used to prepare alcohols from aldehydes and ketones?
Sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4)
- can undergo hydrogenation in the presence of a metal
catalyst although more forcing conditions are generally required (higher temperature and pressure), so this method is rarely used by synthetic chemists.
Compare the reactivity of NaBH4 and LiAlH4.
LiAlH4 is a stronger reducing agent than NaBH4. LiAlH4 can reduce a wider range of functional groups, including carboxylic acids and esters, while NaBH4 is more selective for aldehydes and ketones.
Diols can be prepared from diketones via reduction using _____________ any of the reducing agents
2 equivalents
Diols can also be made via __________________ of an alkene.
anti or syn dihydroxylation
