Bond Lengths and Kekulé Structure Disproof Quiz

Hybridisation

• Hybridisation is the combination of atomic orbitals to give new orbitals which are a mixture of the originals.
• There are different types of hybrid orbitals: sp3, sp2, and others.
• Hybrid orbitals are formed when atomic orbitals combine to form new orbitals with a mixture of s and p characters.
• Hybridisation is important in understanding the structure of molecules, particularly in organic chemistry.

Benzene

• Benzene is a molecule with the formula C6H6.
• The Kekulé structure of benzene shows alternating double and single bonds.
• However, experimentally, it has been found that all bonds in benzene are the same length, which is not consistent with the Kekulé structure.
• The modern structure of benzene shows a ring of delocalised electrons above and below the ring.
• Delocalisation leads to a planar, hexagonal structure with equal bond lengths.

Disproving Kekulé

• Bond lengths: measurement of bond lengths using x-ray diffraction shows that all bonds in benzene are the same length.
• Enthalpy of hydrogenation: the energy released when benzene is reduced to cyclohexane is lower than expected, indicating that benzene is more stable than expected.
• Chemical reactions: benzene does not react with bromine water, which suggests that it does not have alternating double and single bonds.

Reactions of Benzene

• Electrophilic aromatic substitution: a reaction in which an electrophile substitutes a hydrogen atom on the benzene ring.
• Nitration of benzene: a reaction in which benzene reacts with nitric acid to form nitrobenzene.
• Halogenation of benzene: a reaction in which benzene reacts with chlorine or bromine to form a halogenated benzene.
• Friedel-Crafts alkylation: a reaction in which benzene reacts with an alkyl halide to form a alkylated benzene.
• Friedel-Crafts acylation: a reaction in which benzene reacts with an acyl halide to form a acylated benzene.

Molecules of Life

• There are four classes of biological molecules: carbohydrates, lipids, proteins, and nucleic acids.
• Carbohydrates are made up of monosaccharides, which are simple sugars.
• Lipids are made up of glycerol and fatty acids.
• Proteins are made up of amino acids, which are the building blocks of life.
• Nucleic acids are made up of nucleotides, which are the building blocks of DNA and RNA.

Carbohydrates

• Monosaccharides: simple sugars, such as glucose and fructose.
• Oligosaccharides: short chains of monosaccharides, such as sucrose.
• Polysaccharides: long chains of monosaccharides, such as starch, glycogen, and cellulose.
• Carbohydrates are an important energy source for cells.

Proteins

• Amino acids: the building blocks of proteins, which are classified as essential or non-essential.
• Polypeptide chains: amino acids linked together by peptide bonds.
• Primary structure: the sequence of amino acids in a protein.
• Secondary structure: the 3D structure of a protein, including alpha helices and beta sheets.
• Tertiary structure: the overall 3D shape of a protein.
• Quaternary structure: the structure of a protein with multiple polypeptide chains.

Lipids

• Fatty acids: long chains of carbon atoms with a carboxyl group at one end.
• Saturated and unsaturated fatty acids: differ in the presence of double bonds in the carbon chain.
• Phospholipids: lipids with a phosphate group and a hydrophilic head, which are important components of cell membranes.
• Triglycerides: lipids with three fatty acid chains attached to a glycerol molecule, which are important for energy storage.