Organic reactions Flashcards
What is a reaction mechanism?
A reaction mechanism is a detailed description of how old
bonds are broken and new bonds are formed as a starting
material is converted to a products.
What is homolytic cleavage?
Breaking a bond by equally dividing the electrons between the two atoms in the bond is called homolytic cleavage.
What is heterolytic cleavage?
Breaking a bond by unequally dividing the electrons between the two atoms in the bond is called heterolytic cleavage.
Like bond cleavage, bond formation occurs in two different
ways:
1.) Two radicals can each donate one electron to form a bond. Known as radical reactions, since free radicals are involved in forming bonds. Two ions with unlike charges can come together with the negatively charged ion donating both electrons to form the resulting bond.
2.) Negatively charge - electron rich; positive charge -electron poor. These are known as polar reactions.
Radical chain mechanism: (3)
- Initiation: two radicals are formed by homolysis or a sigma bond and this begins the reaction.
- Propagation: a radical reacts with another reactant to form a new sigma bond and another radical.
- Termination: two radical combine to form a stable bond.
Radical reactions INCLUDE: (2)
▪ Halogenation of alkanes (radical substitution reaction).
▪ Polymerisation of alkenes (radical addition reaction).
Polar reactions: (2)
Involve the reaction of an electron-rich centre and an electron-poor centre:
The electron-rich centre is called the nucleophile.
i. This means nucleus loving (Nu-)
ii. Nu- is a fully or partially negatively charged site.
iii. It has got electrons to give.
The electron-poor site is called the electrophile.
i. Electrophile (E+) means electron-loving.
ii. E+ is a fully or partially positively charged site.
iii. An electrophile is looking for an electron.
Predicting organic reactions:
What are the types of Radical reactions? (2)
Halogenation of alkanes
Polymerisation of alkenes
Halogenation of Haloalkoanes reaction mechanism:
Polymerisation of alkenes reaction mechanism:
Types of Polar organic reactions: (2)
- Reactions of alkenes and alkynes: Both have nonpolar bonds, no lone pairs, and no vacant orbitals, but rather pi bonds, which is exposed and accessible.
- The pi component is weaker than the sigma, and because of this weak bond, it is easily broken in the presence of electrophiles. This makes unsaturated hydrocarbons far more reactive than saturated hydrocarbons.
What are the types of alkene addition reactions?
What are carbocations?
Carbocations: forms as an intermediate in addition reactions with alkenes and alkynes.
What is Markovnikov’s rule? (2)
○ Markovnikov’s rule: In the addition of HX to an alkene, the H attaches to the C where there are more H’s and the X attaches to the C with less H’s (more alkyl substituents).
○ The reason for this is that the reaction proceeds via the more stable carbocation intermediate.
Addition reactions to unsymmetrical alkenes: (2)
What are the reactions of alkynes?
What are saturated heteroatoms?
Types include haloalkanes, alcohols, ethers, amines and organometallics.
What type of molecules are saturated heteroatoms? (4)
i. Saturated heteroatom compounds are polar molecules.
ii. The reactivity depends on the carbon-heteroatom bond.
iii. All the heteroatoms that will be considered have a polar negative charge, meaning they are more electronegative than carbon itself and will carry a partial negative charge.
iv. C-X bonds are typically weaker than C-H bonds. Carbon can be both electrophilic or nucleophilic, depending on the substituent (metal or halogen).
Electrophilic and Nucleophilic sites in saturated heteroatoms:
Reactions of alkyl halides: (2)
Nucleophilic substitution reactions:
Nucleophilic elimination reactions:
Substitution and elimination reactions are
always in competition with each other: What factors contribute to the outcome of both reactions? (3)
Temperature:
i. High temperatures favour eliminations.
ii. Hot concentrated NAOH favours elimination.
iii. Lower temperatures favour substitution.
Size of the nucleophile
i. Substitution is favoured for small nucleophiles, as it can enter the molecule and attack the carbon inside the molecule.
ii. Elimination is favoured by large nucleophiles, as it attacks the hydrogen on the surface.
Steric hindrance:
i. I.e. steric hindrance occurs when the atoms in a compound molecule are arranged to create repulsion between the substituents.
ii. Absence of steric hindrance (small hydrogen atoms around the carbon centre), favours substitution.
ii. A sterically hindered carbon or a crowded carbon centre (bigger groups around carbon), favours elimination.
What are the classifications of alcohol?
a. Compounds that have the hydroxyl functional group(-OH) bonded to a saturated carbon atom.
b. Classification of alcohols: alcohols are either primary, secondary or tertiary depending on the number of carbon substituents bonded to the hydroxyl group.
c. Reactions of alcohols: can undergo substitution and elimination reactions (dehydration and
oxidation reactions).
d. -OH is not as good a leaving group as the halogens (C-OH bond is very strong),
Reaction mechanism of substitution reactions with alcohols:
Reaction mechanism of elimination (regular) reactions with alcohols:
Reaction mechanism of elimination (oxidation) reactions with alcohols:
What are the classification of amines? (3)
a. Amines are organic bases derived from ammonia. b. Classified as either primary, secondary or tertiary, depending on the degree of substitution at the nitrogen atom.
Compounds with 4 groups attached to the nitrogen are known, but N-atom carries a _____ charge (quaternary cations) - from Formal charge. Exists as ____.
positive
salts
Unsaturated heteroatom compounds includes:
What are the types of reactions for aldehydes and ketones?
How do nucleophilic addition reactions take place?
How is the reduction of aldehydes and ketones achieved?
What does the oxidation of aldehydes entail?
Reactions of composite functional groups: (2)
a. The double bond leads us to predict an addition reaction.
b. The heteroatom leads us to predict a substitution or elimination.
c. Remember addition and elimination result in substitution, so overall we see a substitution reaction.
General representation of substitution reactions of composite functional groups.
Substitution reactions with composite functional groups:
Acid-base properties of composite functional groups:
Reduction reactions of composite functional groups:
Benzene family:
Simplest neutral aromatic compound, and its
derivatives:
What reactions does benzene undergo?
Electrophilic substitution of benzene:
Halogenation addition of a halogen of benzene:
What are the Friedel Crafts reactions for benzene? (2)
How are condensation polymers formed?
What are addition polymers?
What are biopolymers?
What are the types of biopolymers? (4)
What are zwitterions?
In aqueous solvents at specific pH (for each amino acid) the amino acid exists mainly as a zwitterion. This means they have one positive charge and one negative charge and are neutral overall, but dipolar. This pH is called the
______ point.
isoelectric
Amino acids can link together to form long chains to form proteins. Proteins are polymers made up of repeating amino acid units.
They link by means of ____ ____: the amine group on one amino acid links to the carboxylic acid of another.
Shorter chain of amino acids are called peptides and longer chain are called polypeptides.
amide bond
Nucleic acids and DNA:
