Molecular Geometry Quiz

General and Inorganic Chemistry PHCM101

• The lecture is titled Synthetic and Natural Organic Polymers
• The lecture is part of a series, numbered as lecture 12
• The lecturer's name is Dr. Nesrine El-Gohary

Competencies

• Define polymers and different types of polymerization reactions
• Differentiate between different stereoisomers
• Demonstrates understanding of different types of polymers (natural and synthetic)
• Differentiate between thermoplastic and thermoset polymers

Polymers

• A polymer is a large molecule (macromolecule) of high molecular weight made up of many repeating chemical units called monomers.
• Naturally occurring polymers include proteins, nucleic acids, cellulose, and rubber.
• Synthetic polymers include Dacron, Nylon, Teflon, and Kevlar.

Classification of Polymers

• Polymers are classified according to their synthesis method (addition or condensation), origin (natural or synthetic), and physical properties (thermoplastic or thermoset).

Revision on some basic organic Chemistry concepts

• Alkanes: Simplest organic molecules, consisting only of carbon and hydrogen atoms with only single bonds (sigma bonds).
• Alkanes List: Methane, Ethane, Propane, Butane, etc. A list by number of carbon atoms is given.
• Alkenes: Hydrocarbons containing at least one carbon-carbon double bond (molecule contains a pi bond).
• Alkynes: Hydrocarbons containing at least one carbon-carbon triple bond (molecule contains two pi bonds).

Free Radicals

• A molecule or atom having an unpaired valence electron is called a free radical.
• Free radicals are highly reactive.
• Free radicals are involved in addition polymerization.
• Free radicals attack weak bonds (pi bonds) in alkenes or alkynes.

Functional Groups

• Carboxylic group: A-C(=O)OH
• Amino group: -NH2
• Amide group: -C(=O)NH-
• Alcohol group: -OH
• Ester group: -C(=O)O-

Synthesis of Polymers

• Addition Polymerisation: Monomers add together to form the polymer without any loss of atoms. A carbon-carbon double or triple bond is needed in the monomer.
• Ethylene (ethylene monomer): Forms Poly(ethene) (polyethylene) with addition polymerization

Mechanism of addition polymerization

• Initiation: An initiator breaks a double bond.
• Propagation: Chain growth occurs.
• Termination: Chain growth stops.

Addition Polymerisation

• The polymer is the only product.
• Reaction proceeds by a free radical mechanism.
• Involves the opening out of a double/triple bond, where the free radical attacks the pi bond.
• Conditions are high pressure, temperature and an initiator (to provide the initial free radical).
• Peroxides are often used as initiator/catalyst(source of free radical).
• Conditions of the reaction can alter the properties of the polymer.

Homopolymer

• Polymers made up of only one type of monomer

Copolymer

• Polymers made up of two or more monomers

Poly(propene) and stereoisomerism

• Stereoisomers: Molecules with the same atoms, connected in the same sequence, but atoms are positioned differently in space.
• Isotactic: A very regular type of polymer chain; all methyl groups are on the same side.
• Syndiotactic: A slightly less regular but still very ordered polymer; methyl groups alternate sides of the chain.
• Atactic: A completely random allocation of methyl groups, weakest polymer, lowest melting point.
• Degree of randomness affects polymer strength and melting point.
• Catalysts can direct polymerization towards more ordered forms.

Synthesis of polymers (Condensation polymers)

• Involves two or more monomers with different functional groups.
• Elimination of water or another small molecule.

Polyesters

• Dacron is a polyester.
• A polymer of benzene-1,4-dicarboxylic acid and ethane-1,2-diol (ethylene glycol).
• Ester linkage is formed between monomers.

Polyamides

• These polymers involve the linkage of two monomers (amine and acid) through the amide linkage (as in nylon).
• Nylon 6,6 is a specific type of polyamide.

Kevlar polyamide

• Kevlar is a polyamide.
• Polymer of benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene

Uses of Polyesters and Polyamides

• Main use is as fibers in clothing.
• Often blended with natural fibers (like cotton) to give desired properties.

Natural and Synthetic Polymers (Proteins)

• Condensation polymers, repeating units are amino acids
• All amino acids contain at least one carboxyl group and one amino group.
• Proteins are polyamides, specifically called polypeptides.
• Key role in biological processes (enzymes, hormones, supports, protection)

Natural and Synthetic Polymers (Nucleic acids)

• Condensation polymers, repeating units are nucleotides.
• Nucleic acids are polynucleotides.
• Contain information for building and functioning of an organism (DNA & RNA)

Natural and Synthetic Polymers (Cellulose)

• Condensation polymer, long chain of repeating glucose units.
• Cellulose is a polysaccharide.
• Major component of woody plants and cotton.
• Useful in textiles and paper production.
• Can be chemically modified (e.g., with nitric acid to make explosives).

Natural and Synthetic Polymers (Rubber)

• Addition polymer, repeating units are a monomer called isoprene.
• Natural rubber is obtained from latex of rubber trees.
• Latex is an aqueous suspension of rubber particles.
• Rubber is described to be:
• Tough
• Elastic
• Soft and sticky when hot.
• Vulcanization (linking chains with sulfur) improves properties.

Synthetic Polymers

• Polyethene, polypropylene, nylon, Dacron, Kevlar, and styrene-butadiene rubber.

Thermoplastic and Thermoset polymers

• Thermoplastic: Polymer that can be softened by heating and then molded into desired shapes.
• Thermoset: Polymer that cannot be softened by heating and so cannot be remolded.

Exercise (multiple choice questions)

References

• Raymond Chang, Chemistry, 11th Edition, McGraw-Hill Companies.

General and Inorganic Chemistry (PHCM101) Lecture 7 (Bonding; General Concepts II)

• Valance-Shell Electron Pair Repulsion Model (VSEPR): Explains molecular shapes by electron-pair repulsions.
• Predicting effect of unshared/multiple bonds on geometry
• Predicting the polarity of molecules

Recap: Drawing Lewis structures

• Steps for drawing Lewis structures

Molecular Geometry

• General shape of a molecule, three-dimensional arrangement of atoms
• Diatomic molecules (linear)
• Polyatomic molecules (bond angles)
• Effect of lone pairs

Molecular Geometry (multiple bonds)

• Multiple bonds occupy the same space region as single bonds
• Multiple bonds do not affect molecular geometry.

Molecular Geometry (effect of lone pairs)

• How lone pairs affect molecular geometry.
• Electron-pair geometry vs. molecular geometry

Valence Bond Theory

• Atomic orbitals combine to form molecular orbitals for bonding.
• Hybridization: Mixing of native atomic orbitals to create special orbitals for bonding (e.g., sp3, sp2, sp)
• How to determine hybridization from the number of effective electron pairs. (e.g lone pairs, single, double, triple bonds)

Additional Topics

• Various topics relating to covalent and sp, sp2 and sp3 Hybridization for different kinds of chemical species
• Types of Covalent bonds (sigma and pi)

Test your knowledge on molecular geometries and bond angles with this quiz. Explore topics such as electron pair repulsion, lone pairs, and the characteristics of sigma and pi bonds. Perfect for chemistry students looking to reinforce their understanding of molecular structures.