Reactivity 3.3: Organic Chemistry~hydrocarbons

Define Homologous Series

A series of compounds of the SAME family, with the same general formula, which differ by a common structural unit

• successive members differ from each other by −CH2−. group

• They show a gradual change in physical properties (for example their melting-/boiling points increase as a result of increasingly stronger London dispersion forces).

What are Alkanes? What is its Formula

Homologous series of SATAURATED hydrocarbons containing carbon-carbon single bonds

Formula: C_nH_2n+2

What are Alkenes? What is its Formula?

Homologous series of UNSATURATED carbon-carbon-carbon double bonds

Formula:C_nH_2n

What are Alkynes? What is its Formula?

Homologous series of UNSATURATED carbon-carbon-carbon triple bonds

Formula:C_nH_2n-2

What are Aromatic Compounds

Contain at least 1 benzene ring

What are Aliphatic Compounds

Compounds WITHOUT benzene rings

Be able to Identify Functional groups and structures

Define Homolytic bond fission

• A covalent bond between 2 atoms in a molecule breaks with each atom taking 1 electron from the bond

• Results in free radicals, which are HIGHLY reactive as they are unpaired

Define Heterolytic Bond fission

• Covalent bond between 2 atoms in a molecule breaks with on atom taking both bonding electrons

• Results in cation/anions

Why do Alkanes have LOW reactivity

• Due to strong C-C and C-H bonds and they are non-polar which makes them unreactive to common(polar) reactants

What reaction do alkANES undergo?

• Free radical substitutions (halogenation)

• Combustion

Formula for Complete Combustion

C_xH_y + O₂ —> CO₂ + H₂O

• When oxygen supply is sufficient

Formula for Incomplete Combustion

C_xH_y + O₂ —> CO + H₂O

• When Oxygen supply is limited

• Carbon is sometimes produced in extreme conditions

What are the 3 processes Alkanes go through to undergo halogenation

• Initiation

• Propagation

• Termination

Describe Initiation

• UV light is needed to break the bond through homolytic fission to produce free radicals

Describe Propagation

• Series of rxn between radicals to produce non-radicals and free radicals

• the number of free radicals stays the same

Describe Termination

• Free radicals react with each other to form molecules and reaction stops when there are no more radicals

• Number of free radicals decreases

Why do alkenes have higher reactivity than alkanes

• Because double bonds are the site of reactivity

• pi bonds have electron density farther nuclei, so it is more easily broken

What reaction do AlkENES

Electrophilic addition rxn, where 2 molecules combine to produce larger molecule

Describe the steps of electrophilic addition steps

1. Molecule being added (e.g. halogen) NEEDS to be polarized(slightly (+), slightly (-)). Allows electrons in molecule to be repel electron-rich double bond as molecule approaches it

2. Double bonded carbons is attached by atom with partial positive charge, and the molecule splits HETEROLYTICALLY (creates a negative atom).

3. Positive atom is attached and adjacent carbons become + charged (carbocation intermediate)

4. Unstable carbocation and anion form a bond creating a stable product

what reactions do alkenes undergo

• Hydrogenation (hydrogen gas reacts with alkene)

• Hydration

• Halogenation(Alkenes react with halogens to produce disubstituted halogenoalkanes)

• Hydrohalogenation

Describe hydrohalogenation (symmetric addition)

• Hydrogen halides react with alkenes to produce monosubstituted hydrogenoalkanes

• H—X is polar molecule that undergoes heterolytic fission as well to form H⁺ and X^- . Therefore molecule does NOT need to be polarized first

• H+ (electrophile) attacks double bond, then carbocation intermediate reacts with X^-.

• Reactivity of hydrogen halides: HI>HBr>HCl(bonds increases in strength as reactivity decreases)

Describe asymmetric addition reaction

• When an asymmetric alkene (such as propene CH2 −−CH−CH3 ) reacts with an asymmetric reactant (such as H X or H OH), two structural isomers are formed from two different competing reactions.

• According to Markovnikov’s rule: the + δ H or + δ X gets attached to the carbon with more hydrogen substituents, and the halide − δ X attaches to the carbon with more alkyl substituents (more stabilised carbocation aka secondary carbocation).

Describe hydration

• Reaction between water and alkene to form alcohol

• Uses heat and concentrated sulfuric acid as catalyst

• This electrophilic addition produces wither 1 or 2 products (depending if alkene is symmetric or asymmetric)

Describe hydration (electrophilic addition of water) steps

1. The double C=C bond is broken because it is attacked by H+ present in sulfuric acid, which converts into a C-C single bond, making a carbocation.

2. Water then acts as nucleophile(electron-rich) and attacks the carbocations, forming the alcohol

Describe polymerization of alkenes

• Formed when smaller unsaturated molecules aka alkenes (monomers) react together

• In addition polymerization, small monomers that contain a C=C double bond link together to form a longer polymer

• During the process the double bonds in the monomers are converted into single bonds in the polymer