IB SL Chemistry - Topic 10

What is a Homologous Series?

What is a Homologous Series?

A homologous series is a series of compound from the same family, with the same general formula, which differ from each other by a common structural unit (usually -CH2-)

Features of a Homologous Series (5)

1. Successive members of a homologous series differ by a -CH2- group

2. Members of a homologous series can be represented by the same general formula

3. Members of a homologous series contain the same functional group

4. Members of a homologous series have similar chemical properties

5. Members of a homologous series show a gradation in their physical properties

Why do members of a Homologous Series show a gradation in physical properties?

• Given that successive members of a homologous series differ by a -CH2- group, they have successively longer carbon chains, and therefore successively larger molar masses

• As the molar mass of a molecule increases, there are more e-, therefore more temporary dipoles and therefore, the LDF between the molecules are stronger

• These stronger LDF require more energy to break, meaning successive members have increasing boiling points

What is a Functional Group?

Functional groups are the atom/group of atoms in a molecule that give it its characteristic chemical properties. It is the reactive parts of molecules.

Saturated vs. Unsaturated Compounds

Saturated: Compounds which contain only single bonds

Unsaturated: Compounds which contain double and triple bonds

What is a Hydrocarbon?

A compound consisting of only carbon and hydrogen

Alkanes

FORMULA***: C_nH_{2n + 2}

Prefix: N/A

Suffix: -ane

Functional Group Name: alkyl

Alkenes

FORMULA***: C_nH_2n

Prefix: N/A

Suffix: -ene

Functional Group Name: alkenyl

Alkynes

FORMULA***: C_nH_2n-2

Prefix: N/A

Suffix: -yne

Functional Group Name: alkynyl

Alcohols

FORMULA***: C_nH_2n+1OH

Prefix: hydroxy

Suffix: -OL

Functional Group Name: hydroxyl

Aldehydes

FORMULA***: C_nH_2nO (C_nH_2n+1-CHO)

Prefix: formyl-

Suffix: -AL

Functional Group Name: aldehyde (formyl) (carbonyl)

Ketones

FORMULA***: C_nH_2nO

Prefix: oxo-

Suffix: -ONE

Functional Group Name: ketone (oxo)(carbonyl)

Carboxylic Acids

FORMULA***: C_nH_2nO₂ (C_nH_2n+1COOH)

Prefix: N/A- (carboxy)

Suffix: -OIC ACID

Functional Group Name: carboxyl

Esters

FORMULA***: N/A (R-COO-R’)

Prefix: N/A- (acyloxy)

Suffix: -OATE

Functional Group Name: carboNyl

Amines

FORMULA***: N/A

Prefix: amino

Suffix: -AMINE

Functional Group Name: amino

Amides

FORMULA***: N/A

Prefix: amido

Suffix: -AMIDE

Functional Group Name: carbox amide

Nitriles

FORMULA***: N/A

Prefix: CYANO

Suffix: -NITRILE

Functional Group Name: nitrile

Ethers

FORMULA***: C_nH_2n+2O

Prefix: ALKOXY

Suffix: -ether

Functional Group Name: ethers

Arenes (Benzene)

FORMULA***: N/A

Prefix: PHENYL

Suffix: -benzene

Functional Group Name: benzene

Boiling Points of Organic Compounds (lowest to highest)

Alkane, halogenoalkane, aldehyde, ketone, amine, alcohol, carboxylic acid

Molecular Formula

Actual number of atoms present in a molecule

Eg) C₂H₆

Empirical Formula

Simplest whole number ratio of atoms in the compound

Eg) CH₃

Full Structural Formula

Shows all bonds between all atoms

Condensed Structural Formula

Bonds between atoms are omitted

Skeletal Formula

All atoms are omitted leaving only the carbon backbone

Primary, Secondary and Tertiary Compounds

PRIMARY: A primary carbon is attached to the functional group and zero or ONE other carbon atoms (2 H)

SECONDARY: A secondary carbon is attached to the functional group and TWO other carbon atoms (1 H)

TERTIARY: A tertiary carbon is attached to the functional group and THREE other carbon atoms (0 H)

Primary, Secondary and Tertiary Amines

What are Structural Isomers?

Structural isomers are compounds which have the same molecular formula but different structural formulae. Different isomers have different chemical and physical properties.

Boiling points of Branched Isomers

Branched-chain isomers usually have lower boiling points than straight-chain isomers. They have lower surface area which reduces the strength of the LDFs.

Aromatic Compounds

These are compounds that have a phenyl functional group (arenes) and are known as aromatics. Aromatics are automatically classified as unsaturated compounds.

Benzene

• Benzene is a cyclic structure where single bonds attach C-atoms to each other and to a H-atom

• The C-atoms form 3 sigma bonds with 120º angles, making it a planar molecule

• The unhybridized p-orbitals of each carbon atom overlap perpendicularly to the benzene ring, thus forming a pi electron cloud in which the e- density is concentrated above and below the plane of the benzene ring

Kekule’s Benzene Structure

Kekulé was the first to suggest a sensible structure for benzene. The carbons are arranged in a hexagon, and he suggested alternating double and single bonds between them.

It showed benzene to have alternating single and double bonds, but it was proved incorrect and instead a circle is drawn in the middle (resonance).

Chemical Evidence Going Against Kekule’s Benzene Structure

• Benzene undergoes electrophilic substitution reacts and resists addition reactions

• If benzene had 3 Carbon double bonds, as Kekule hypothesized, it would undergo additional reactions

• However, due to the real resonance structure of benzene, addition reactions would be energetically not favoured as they would disrupt the cloud of delocalized e-

• The hydrogenation of benzene is less exothermic than the hydrogenation of Kekule’s structure ((-210 K)/mol vs (-362 K)/mol)

• This is because the delocalization in bbenzene reduces repulsion between e- and thus makes benzene more stable

• The additional energy (152 kJ/mol) would be required to overcome the stability of the delocalization (known as the resonance energy)

Physical Evidence Against Kekule’s structure

BOND LENGTHS

• All C-C in benzene are of equal lengths, intermediate between single and double bonds

• If benzene contained alternating single and double bonds, we would expect the bonds to be of different lengths

• Instead, bonds in benzene have a bond order of 1.5 (due to e- delocalization)

ISOMERS

• Unlike Kekules structure, benzene only has one isomer of disubstituted benzene compounds (like 1,2-dibromobenzene)

• This is because all of the C-C bonds are identical

• If benzene contained alternating single and double bonds then we would expect 2 isomers

What is an Addition Reaction?

• Occurs when two reactants combined to form a single product

• Characteristic of unsaturated compounds (double bonded and triple bonded)

• Involves breaking of double and/or triple bonds

What is an Elimination reaction?

Inolves splitting an organic molecule into an unsaturated hydrocarbon (alkene or alkyne) and a small inorganic molecule. (GOING FROM ANE TO ENE)

What is a Substitution reaction?

When one functional group on an organic molecule is substituted for another.

Special Types:

• Aromatic Compounds (only undergo substitution)

• Nucleophilic Substitution (when halogens are substituted by hydroxyl)

• Free Radical Substitution (when hydrogen is substituted by a halogen)

What is an Oxidation reaction?

A reaction where a C atom makes more bonds to O and fewer bonds to H. To achieve oxidation of an organic compound, it must be mixed with an oxidizing agent in an acidic solution.

Oxidizing agents: KMnO₄ (starts dark purple and turns very light pink) and Na₂Cr₂O₇ (starts orange and turns green)

Acidic solution: H₂SO₄

What is a Reduction reaction?

A reaction where a C atom makes more bonds to H and/or fewer bonds to O. To achieve reduction of an organic compound, it must be mixed with a reducing agent.

Reducing agents: LiAlH₄ (lithium aluminium hydride) and NaBH₄ (sodium borohydride)

Fractional distillation vs. Reflux conditions

Fractional distillation (half-way oxidation or reduction - eg going from alcohol to aldehyde)

Reflux conditions (full oxidation or reduction - eg going from alcohol to carboxylic acid)

What is a Condensation reaction?

A reaction where 2 organic molecules are joined by removing a H2O molecule (water is a bi-product). (EG - esterification)

What is a Hydrolysis reaction?

Using an H2O molecule to split a large organic molecule into 2 smaller ones.

• Ester + water —> carboxylic acid + alcohol

• Amide + water —> carboxylic acid + amine

Reactivity of Alkenes

• Alkenes are more reactive than alkanes due to the C=C bond

• The PI bond in the C=C bond is easily broken, allowing alkenes to undergo addition reactions

Hydration of Alkenes

• Alkenes react with water in the presence of an acid catalyst to produce alcohols

• The acid catalyst may either be concentrated H2O4 or H3PO4

What reactions do Alkanes undergo?

Combustion, Cracking, Substitution (Halogenation)

What reactions do Alkenes undergo?

Addition reactions (halogenation, hydrogenation, hydration, hydrogen halidation, oxidation)

Hydrogenation of Alkenes

Alkenes react with H2 in the presence of a Nickel catalyst at about 150ºC

Halogenation of Alkenes

Alkenes react with halogens to produce dihalogeno compounds. These reactions occur readily at room temperature and are accompanied by the loss of the colour of the halogen.

Hydrogen Halide Addition of Alkenes

• Alkenes react with hydrogen halides to produce halogenoalkanes

• All the hydrogen halides can react in this way but the ones with weaker bonds react more readily. Te hydrogen halide bond decreases in strength down group 17

Polymerization of Alkenes

Alkenes can undergo addition polymerization to join together and form addition polymers

Economic importance of Alkene reactions

POLYMERIZATION

• Synthesis of plastics which are cheap and versatile

MANUFACTURE OF MARGARINE

• Oils containing unsaturated hydrocarbons are hydrogenated to form margarine

MANUFACTURE OF ALCOHOLS

• Hydration of alkenes forms alcohols

Low reactivity of alkanes

• The C-C and C-H bonds are non-polar bonds, so alkanes are not susceptible to attack by most reactants

• The C-C and C-H bonds are strong covalent bonds, meaning that alkanes will only react in the presence of a strong enough energy source to break these bonds (EA)

What is heterolytic fission?

• Occurs when a covalent bond splits into 2 oppositely charged ions (one + and one -)

• THIS IS WHAT WE ARE USED TO

What is homolytic fission?

• Occurs when a covalent bond breaks and each atom gets one electron

• This produces 2 free radicals (highly reactive species with an unpaired electron)

What is a Free-Radical?

• Highly reactive species with an unpaired electron

What is Free-Radical Substitution?

• Only occurs in the presence of UV light

• In this rxn, another reactant (eg. Cl) takes the place of a H atom in the alkane

• This rxn required the energy from UV light to break the covalent bond in Cl₂ (photochemical homolytic fission) to form Cl free radicals

What are all of the stages and steps of Free-Radical substitution?

1. Initiation

   1. UV light is sued to cause the homolytic fission of the diatomic halogen. This creates 2 free radicals. [PHOTOCHEMICAL HOMOLYTIC FISSION]

      1. Cl-Cl —> •Cl + Cl•

2. Propagation

   1. Consumes a free radical, BUT also produces a free radical so the rxn can continue

      1. •Cl +HCH3 —> HCl + •CH3

      2. •CH3 + Cl-Cl —> ClCH3 + •Cl

3. Termination

   1. Consumes 2 free radicals and produces neutral, unreactive molecules

      1. •Cl + •Cl —> Cl₂

      2. •CH3 + •CH3 —> CH₃CH₃ (ethane)

      3. •Cl + •CH3 —> ClCH₃

Why would free radical substitution mechanism give a poor yield of the desired product?

This mechanism rarely results in the intended halogenoalkane because the substitution of the halogen is totally random and can happen to any hydrogen on any molecule in the rxn mixture.

Further substitutions would occur, thus giving a mixture of many different products and so a low probability of the desired product.

Bromine water unsaturation test

• When Br water is added to alkenes, an addition reaction occurs. The alkene decolourizes the brown colour of the Br water to COLOURLESS (not clear******).

  • ALKENE = TURNS IT COLOURLESS

• Alkanes do not react readily with Br water as the presence of UV light is required. Thus, when Br water is added, NO REACTION OCCURS and there is NO COLOUR CHANGE.

  • ALKANE = IT REMAINS BROWN

Br Water = Br_(aq)

Oxidation of Alcohols

• Alcohol molecules can react with oxidizing agents which selectively oxidize the ol-carbon

  • Eg) potassium permanganate, potassium dichromate, sodium dichromate

Oxidation of Primary Alcohols

• Primary alcohols are oxidized in a two-step reaction that first produces an aldehyde which is further oxidized to a carboxylic acid

• If the aldehyde is the required product, it can be removed from the rxn mixture by fractional distillation

  • This is possible as aldehydes have a lower boiling point than alcohols and carboxylic acids because they don’t have H-bonding between reactants

  • Bi-product: water

• If the carboxylic acid is the required product, we must leave the aldehyde in contact with the oxidizing agents for a prolonged period of time (REFLUX)

  • This required heating under reflux

Oxidation of Secondary Alcohols

• The oxidation of secondary alcohols is a one-step process that produces a KETONE

• After the ketone is produced, no further oxidation is possible as there are no more H-atoms attached to the ol-carbon

• Bi-product: water

Conditions for oxidation reactions of alcohols

ALDEHYDE

• Acidified oxidizing agent, heat, use excess alcohol & distill immediately

CARBOXYLIC ACID

• Excess acidified oxidizing agent, heat, reflux longer

KETONE

• Acidified oxidizing agent, heat, reflux

Esterification

• Alcohols react with carboxylic acids to form esters in a condensation reaction

• The reaction is an EQUILIBRIUM RXN and is catalyzed by conc. H2SO4

• Unlike carboxylic acids and alcohols, esters CANNOT form H-bonds between molecules so they have lower boiling points

  • Thus, they can be seperated by distillation

• They are mostly insoluble in water (form a layer on the surface)

• REQUIRED HEATING UNDER REFLUX

Use of Esters (3)

• Food flavouring (synthetic)

• Fragrant odours for perfume

• Make surfactants (eg. soap)

Combustion Reactions of Alkanes

COMPLETE COMBUSTION

• Alkanes burn in the presence of excess O2 to produce CO2 gas and H2O gas

INCOMPLETE COMBUSTION

• When the O2 supply is limited, the products are either CO gas and H2O gas or, when O2 is extremely limited, SOLID carbon and H2O gas

Combustion of Alcohols

• Like alkanes burn in excess O2 to produce CO2 gas and H2O gas

• The enthalpy of combustion of alcohols increases up the homologous series as the number of CO2 molecules produced increases

Physical Properties of AMINES

MP/BP

• Primary and secondary amines have higher MP and BPs since they have H-bonds between molecules (N-H)

• Longer amine chain = stronger LDF = higher melting + boiling points

SOLUBILITY

• Water can form H-bonds with all amines (including teriary) meaning that all amines are SOLUBLE IN WATER

• The longer the amine chains are, the less soluble they come as the chain becomes non-polar

Production of Amides

Carboxylic Acid + Amine < — > Amide + Water

• It is a condensation rxn that requires conc, H2SO4 AS A CATALYST

• Primary and secondary amines can undergo this rxn (not 3º)

• ITS A REVERSIBLE RXN (hydrolysis can take place of amides)

Markovnikov’s Rule

When more than one product is possible, the carbon with most electron bonds receive it

• “The rich get richer”

Naming Polymers

Find the MONOMER unit and name it. Add “poly” in front of that name.

Bond Strengths and Lengths

Strengths: Triple > Double > Single

• Triple is most strong while single is most weak

Lengths: Single > Double > Triple

• Triple is shortest while single is longest

What is a Nucleophile?

A fully negative ion or polar molecule that is attracted to the nucleus of another atom.

• Eg) OH- , CN-, H2O, NH3

What is the Transition State Complex/Activated Complex?

A point in time where bonds are breaking at the same time as they are forming

What is steric interference?

It prevents or slows down a chemical reaction, caused by the arrangement of atoms in a molecule.

• It makes it harder for the rxn to take place

What is SN2?

• It has 2 steps involved in its RDS

• It involves the formation of a transition state complex

• 0º, 1º and 2º experience this one (although 2º do it badly)

What is SN1?

• It has 1 step involved in its RDS

• It involves the formation of a carbocation

• 3º experience this

What is the speed of each reaction in Nucleophilic substitution?

3º>0º>1º>2º

What is Condensation Polymerization?

• Condensation rxns are used to join together a series of monomers that contain 2 functional groups; WATER forms as a bi-product

  • POLYESTER = dioic acid + diol

  • POLYAMIDE = dioic acid + diamine

What is fractional distillation?

Fractional distillation is a process by which components in a chemical mixture are separated into different parts (called fractions) according to their different boiling points. Fractional distillation is used to purify chemicals and also to separate mixtures to obtain their components.

What is reflux?

Reflux involves heating the chemical reaction for a specific amount of time, while continually cooling the vapour produced back into liquid form, using a condenser. The vapours produced above the reaction continually undergo condensation, returning to the flask as a condensate.

What is electrophillic addition

Number of Structural Isomers:

Methane: ?

Ethane: ?

Propane: ?

Butane: ?

Pentane: ?

Hexane: ?

Heptane: ?

Octane: ?

Nonane: ?

Decane: ?

Methane: 0

Ethane: 0

Propane: 0

Butane: 2

Pentane: 3

Hexane: 5

Heptane: 9

Octane: 18

Nonane: 35

Decane: 75

Class vs. functional group (example?)

Class: alcohol

Functional group: hydroxyl

Arenes

Aromatic compounds contain a benzene ring <that forms resonance > which is known as a phenyl functional group

Why do isomers have lower BP/MPs?

The branching of a chain produces a more spherical shape to the molecule. There is less surface contact between the molecules than with straight-chain isomers. Thus, branched-chain isomers have weaker intermolecular forces and, consequently, lower boiling points.

Strength and IMF trend in functional groups

The solubility of organic compounds in water depends on their ability to ________________________________________________

The solubility of organic compounds in water depends on their ability to form hydrogen bonds with water molecules.

Hydrophobic (def)

The term hydrophobic refers to the non-polar hydrocarbon chain of a molecule which is insoluble in polar solvents such as water. 

Benzene is a known ____________, so inhalation of benzene vapors is definitely not recommended.

CARCINOGEN

Carbon monoxide characteristics

Note that carbon monoxide is a highly poisonous gas, with no taste, smell or colour. It binds strongly to haemoglobin in the blood, which leads to suffocation and eventually death if it is inhaled in sufficiently high concentrations.

Incomplete combustion is characterised by…

This is known as incomplete combustion and is characterised by the appearance of a yellow or orange flame.

Alkenes (Hydrogenation)

Catalyst: ?

Conditions: ?

Real-life: ?

H₂ IS ADDED TO CARBON DOUBLE BOND

Catalyst: Nickel (Ni) catalyst.

Conditions: High pressure and high temperature (180º)

Real-life: Maragarine production

Alkenes (Hydration)

Catalyst: ?

Conditions: ?

Real-life: ?

STEAM (H2O_(g)) IS ADDED TO ALKENE

Catalyst: H₂SO₄

Conditions: -

Real-life: To produce alcohols

Alkenes (Halogenation)

Catalyst: ?

Conditions: ?

Real-life: ?

Alkenes react with halogens to produce dihalogenoalkanes.

Catalyst: ?

Conditions: Room temperature and are indicated by the decolourisation of the reacting halogen

Real-life: ?

Polymers real-life application

Poly(chloroethene), more generally known as PVC (polyvinyl chloride)

• Construction materials, packaging and covers for electrical cables.

Poly(ethene)

• Plastic bags, bowls, bottles, packaging

Alkene summary

Alcohols (Oxidation)

Catalyst: ?

Conditions: ?

Real-life: ?

Catalyst: Oxidizing Agents

• Potassium manganate(VII)

• Acidified potassium dichromate(VI) (Cr₂O₇²⁻/H⁺).

  • Orange solution which changes to a green colour (Cr³⁺ ion)

• Potassium manganate(VII) solution

  • Purple to colourless (Mn²⁺)

Conditions: ?

Real-life: ?

How can you tell any oxidation rxn is complete using Dichromate?

The dichromate ion forms an orange solution which changes to a green colour as it is reduced to the Cr3+ ion

If the aldehyde is the desired product, it can be removed as it forms by ______________.

Explain this process.

If the aldehyde is the desired product, it can be removed as it forms by distillation (Figure 5).

This is possible because the boiling point of the aldehyde is lower than that of both the alcohol and the carboxylic acid. As the aldehyde evaporates and rises up the distillation column, it passes through the condenser where it condenses back to a liquid and runs down into the flask.