Shapes of and Bond Angles in Simple Organic Compounds

A. Miller

Simple Organic Compounds

Ethane

Etnene

Benzene

Simple Organic Compounds

Organic Compounds- consisting of carbon and hydrogen mostly

Recall

Overlapping of atomic orbitals

Formation of sigma bonds

The simple picture of overlap of half-filled atomic orbitals cannot be used to explain the geometry of all molecules especially organic molecules

Hybridisation

The mixing of orbitals Stronger orbitals are created

Methane -CH4

Structure

How Does Methane Forms Four Single Bonds

Ground state configuration 1s2 2s2 2p2

Methane

Needs to have four single bonds

Need four single electrons

Promotion of an electron from the 2s orbital to the 2p orbital

Mixing the 2s and 2p orbitals

Methane

Formation of sp3electronic configuration

Energy is required to do this

Promotion of electron and orbital mixing

hybridization

Mixing of Orbitals

Four single electrons

sp3 Hybrid Orbitals

The promotion electrons followed by the mixing of the orbitals create stronger orbitals

The s orbital mixes with the three p orbitals producing four equivalent sp3 orbitals

Each hybrid orbital contains 25% s character and 75 % p character.

The 4 sp3 hybrid orbitals each contains an electron

Will arrange themselves in a three dimensional space to get as far apart as possible (to minimize repulsion)

Electrons Arrangement in Carbon

Arrangement gives rise to a tetrahedral structure bond angle of 109.5

Overlapping of C and H orbitals

The sp3 hybrid orbitals of carbon overlap with the s orbital of hydrogen containing an electron.

Give rise to the C-H sigma bond

Formation of methane

Overlapping of orbitals

Structure of Methane

C-H bonds

Ethane

overlapping

Etnane

overlapping

Sp2 hybridization

Found in compounds such as alkenes

Formation of bonds to three other atoms (two hydrogens and one carbon)

Each carbon employs a set of sp2 hybrids

sp2 Hybridization

Electron promotion still occur in carbon

Mixing of the 2s and 2p orbitals.

Only two of the p orbitals are mixed with the s orbital.

sp2 Hybridization

The other p orbital remains pure (unhybridized)

Three sp2 hybrid orbitals are created

Two will overlap with hydrogen 1s orbital

Formation of Ethene

The third will overlap with a similar sp2 orbital on the other carbon atom.

Accounting for all the C-H bonds and the C-C sigma bond of the double bond

Each carbon has a pure p orbital containing an electron

Formation of Ethene

The orbitals are perpendicular to the plane of the sp2 orbitals-

Projects above and below the plane

Orbitals close proximity causes overlap sideways forming a pi bond

Ethene

Pi bonds are weaker than sigma bonds

sp2 Hybridization- mixing of orbitals

Hybridized Structure of Ethene

Ethene

Ethene

Benzene

Six carbon atoms in a ring

Shows resonance hybrid

Hexagonal in shape- at each apex there is a carbon bonded to a hydrogen

Benzene

Each carbon is bonded to three other atoms; a hydrogen and two other carbon atoms

Each carbon uses sp2 hybrid orbitals

Each carbon contains a pure p orbital perpendicular to the plane of the ring

Benzene

Each unhybridized p orbital overlaos with two other p orbitals, one on each of the two neighbouring carbon atoms

A large circular pi-type bond is formed above and below the plane

Electrons are delocalized in the benzene ring

Benzene

Overlapping of p orbitals

Benzene

Benzene

Canonical forms

Benzene

Hybridized structure

Structure of solids

Solids can either be

- Amorphous (non-crystalline) or

- Crystalline

Amorphous Solids

Particles have no orderly structure Lack well-defined faces and shapes Many are mixtures of molecules that do not

stack together Most composed of large complicated

molecules Example; rubber, glass

Crystalline Solids

Highly regular/orderly arrangement of atoms, molecules or ions in a crystal.

Usually have flat surfaces, or faces that make definite angle with one another

Example; quartz, diamond

Lattice Structure

Consists of repeating units called unit cell Solid can be represented by a three

dimensional array of points called crystal lattice

Each point in the lattice is called a Lattice points

Lattice Structure

Structural units in the lattice are held by;

- electrostatic forces in ionic crystals

- van der Waals forces in simple

molecular crystals

- hydrogen bonds as in ice

Lattice Structure

Structural units in lattice are held by;

- Covalent bonds as in giant molecular structures as in silicon dioxide (quartz), giant atomic structures as in diamond and graphite

- metallic bond as in metallic crystals such as copper

Ionic Solid-sodium chloride

Face-centred cubic structure

Lattice points are occupied by ions

Each Na+ surrounded by 6 Cl- ions as next nearest neighbour and vice-versa

Strong forces of attraction between oppositely charged ions

Ionic Structure- Sodium Chloride

Blue- chloride ions

Red-sodium ions

Simple Molecular structure

Atoms held by strong covalent bonds

Molecules held by weak van der Waals

forces

Gases or liquids at room temperature

Simple Molecular-Iodine

Atoms covalently bonded in pairs as I2

molecules

Discrete molecules held by weak van der Waals forces

Shiny in appearance due to regular arrangement of molecules

Iodine

Very slightly soluble in water

Dissolve freely in organic solvent

Does not conduct electricity- no separation of charge

Face-centred Cubic Structure

Simple Molecular-Iodine

Molecules in corners and face of unit cell

Giant Molecular-Silicon dioxide

Formed by strong, directional covalent bonds, and has a well-defined local structure

Each silicon atom can bond to four oxygen atoms, giving rise to a giant covalent network structure

Silicon dioxide

Each Si is bonded to four oxygens and each O to two silicon atoms.

The bonding between the atoms goes on and on in three dimensions.

Four oxygen atoms are arrayed at the corners of a tetrahedron around a central silicon atom:

Giant Molecular-Silicon dioxide

Three dimensional structure

Silicon dioxide

bonding

Metallic Structures

Consist entirely of metal atoms.

Usually have hexagonal close-packed, cubic close-packed (face-centred cubic) or body-centred cubic structures

Each atom typically has 8 or 12 adjacent atoms.

Metallic Structure

Bonding due to valence delocalized electrons throughout the entire lattice

i.e. positive ions immersed in a sea of delocalized valence electrons.

Metallic StructureBody-centerd Cubic

There is one host atom (lattice point) at each corner of the cube and one host atom in the center of the cube: Z = 2.

Each corner atom touches the central atom along the body diagonal of the cube

Metallic Structure

Body-centred cubic

Body-centred

Unit cell

Cubic Close-packed/ Face-centred

Arranging layers of close-packed spheres such that the spheres of every third layer overlying one another gives cubic close packing

Cubic Close-packed/Face-centred Cubic

Unit cell has one host atom at each corner and one host atom in each face. 

Each corner atom contributes one eighth of its volume to the cell interior

Each face atom contributes one half of its volume to the cell interior (and there are six faces), then Z = 1/8.8 + 1/2.6 = 4.

Cubic Close-packed/ Face-centred eg. Copper

Face-centred cubic

Hexagonal Close-packed

The unit cell consists of three layers of atoms.

 The top and bottom layers contain six atoms at the corners of a hexagon and  one  atom  at  the  center  of  each hexagon.    

The  middle  layer  contains  three  atoms nestled between the atoms of the top and bottom layers

Hexagonal Close-packed

layers

Giant Atomic Structures

Covalent-network solids

Consist of atoms held together in large network or chains by covalent bonds

Solids are much harder and have higher melting points than molecular solids.

Giant Atomic Structure

Two examples are; diamond and graphite.

Diamond and graphite are two allotropes of carbon

Diamond

Lattice points occupied by carbon atoms

Each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement.

Interconnected three-dimensional array of strong C-C single bonds

Diamond

Diamond is very hard as a result.

Multitude of covalent bonds causes diamond to have a very high melting point 3550 degree Celsius

Does not conduct electricity- no free electrons.

Diamond

Insoluble in water and organic solvents.

No possible attractions which could occur between solvent molecules and carbon atoms which could outweigh the attractions between the covalently bound carbon atoms

Giant Atomic Structure- Diamond

C-C single bonds

Diamond

Tetrahedral arrangement

Giant Atomic Structure- Graphite

Each carbon is covalently bonded to three other in a trigonal planar arrangement.

Each carbon has a single electron that is delocalized and free to move about in the lattice.

Hence graphite conducts electricity along the layers

Giant Atomic Structure- Graphite

Lattice structure consists of layers of interconnected hexagonal rings

Layers are held by weak van der Waals forces

Layers readily slide past each other when rubbed. Giving a greasy feel.

Hence used as a lubricant and in lead pencils

Giant Atomic Structure- Graphite

Insoluble in water and organic solvents - for the same reason that diamond is insoluble.

Attractions between solvent molecules and carbon atoms will never be strong enough to overcome the strong covalent bonds in graphite.

Giant Atomic Structure- Graphite

Layers of carbon atoms

Graphite

Van der Waals forces between layers

Structure of Ice