Molecular Shape and Reactivity Flashcards
What does orbital hybridisation account for?
The bonding of carbon atoms into chains and rings.
Describe the bonding in ethane.
- Two carbon atoms - both are tetrahedral
- C-H bonds formed by overlap of carbon sp3 and hydrogen s orbitals
- C-C bond formed by overlap pf two carbon sp3 orbitals
- Increased bond length for C-C
- Bond angles are near 109.5
Describe the bonds in ethene.
- Each carbon forms four bonds but is bonded to only three atoms
- Hybridisation of an s atomic orbital with two 2p atomic - one p orbital is left non-hybridised
- Electron repulsion minimised by the three sp2 orbitals lying in a plane
- Bond angles close to 120
- Unhybridised p orbital is perpendicular to the plane
Describe how the double bond forms in ethene.
- One bond formed by overlap of one sp2 orbital from each carbon
- second bond formed by side-to-side overlap of the unhybridised p orbitals
How many electrons are involved in the formation of a C=C bond?
four
Compare a C=C bond to a C-C bond.
C=C bond is stronger and shorter than C-C
Why does the rotational barrier exist in ethene?
the p orbitals are well aligned for maximum overlap leading to formation of the pi bond.
Describe the bonds in ethyne.
- forms a triple bond
- hybridisation of only two orbitals - two p orbitals are left non-hybridised
- sp hybrid orbital derived from the combo of one s and one p atomic orbital
- the two sp hybrids are separated by an angle of 180
How many electrons are involved in the formation of a C C triple bond?
six
Compare a C triple bond to a C=C
C triple bond is stronger and shorter than a C=C bond
Describe the bonding in methyl cation.
- Carbon is positively charged and bonded to three hydrogen atoms
- Three orbitals are hybridised
- sp2 hybridisation
- The positively charged C and the three H atoms lie in a plane
- Non-hybridised p orbital remains empty and is perpendicular to the plane
Describe the bonding in methyl radical
- Carbon is bonded to three hydrogen atoms and is sp2 hybridised
- The C and three H atoms lie in a plane
- Radical has one more electron than the cation
- Resides in the non-hybridised p orbital
Describe the bonding in methyl anion
- Carbon is bonded to three hydrogen atoms
- Three pairs of bonding electrons and one lone pair of electrons
- Electron repulsion minimised by adopting tetrahedral geometry
- Negatively charged carbon is sp3 hybridised
- Three bonds formed by sp3 - s overlap
- Lone pair resides in the fourth sp3 orbital of the carbon
Describe the bonding in nitrogen
- Nitrogen hybridises to form four sp3 orbitals
- One of the four sp3 orbitals is occupied by two non-bonding electrons
- N-H bonds formed by sp3 - s overlap
Describe the bonding in amines
C-N bonds formed by sp3-sp3 overlap
- Hydrogens may be replaced by 1/2/3 alkyl or aryl groups
- Electron rich therefore nucleophilic
Describe the bonding in ammonium cations
- Possesses four identical N-H bonds and no lone pairs
- Nitrogen has four sp3 hybridised orbitals
- All bond angles = 109.5
Describe the bonding in oxygen
- O-H bonds formed by sp3-s orbital overlap (water, alcohols)
- C-O bond formed by sp3-sp3 orbital overlap (alcohols, Esthers)
- Lone pairs occupy the remaining two oxygen sp3 orbitals
- Bond angle smaller than in methane (109.5)
Describe the bonding in sulphur atoms
- S-H bonds formed by sp3-s orbital overlap (water, alcohols)
- S-C bond formed by sp3-sp3 orbital overlap (alcohols, Esthers)
- Lone pairs occupy the remaining two sulphur sp3 orbitals
Describe the bonding in hydrogen halides
- Bond formed between s-sp3
(HF = s-2sp3 and HCL = s-3sp3) - Electron density in region s-sp3 overlap decreases as the size of X increases
- HX bond becomes weaker and longer as the X increases
Describe the bonding in Alkyl Halides
- C-X bond formed by sp3-sp3 orbital overlap
- As size of halogen increases sp3 orbital from higher shell used in bond = decreased electron density and longer and weaker bond
Define electronegativity.
- The tendency of an atom to pull bonding electrons toward itself
Define polar covalent bond.
- A covalent bond in which the electron distribution between atoms is unsymmetrical
- Bond polarity is due to difference in electronegativity
Describe the inductive effect.
- An atom’s ability to polarise a bond
- Electron-attracting or electron-withdrawing effect transmitted through s bonds
- Electronegative elements have an electron withdrawing inductive effect
Define dipole moment.
- A measure of the net polarity of a molecule
What are the four main types of interactions in order of increasing strength?
- Van der Waals forces
- Dipole-Dipole interactions
- Hydrogen bonding
- Ion-Ion (Electrostatic)
Describe Van der Waals forces.
- Weak interactions caused by momentary changes in electron density in a molecule
- The only attractive forces present in non polar compounds
- Surface area of a molecule determines the strength of the van der Waals interactions between molecules
Define polarisability (van der Waals)
- A measure of how the electron cloud around an atom responds to changes in its electron environment
Describe dipole-dipole interactions
- Attractive forces between the permanent dipoles of two polar molecules.
Describe hydrogen bonding.
- Non-bonded interaction between a hydrogen atom bonded to an electronegative atom (D) and another electronegative atom (A)
- D/A = O, S, N
- The electron deficient hydrogen is called a hydrogen bond donor and the electron rich heteroatom is called a hydrogen bond acceptor
In ethane which two extreme conformations are possible?
Staggered and eclipsed
What is meant by torsional strain?
- Repulsion felt by bonding electrons as they pass close to the bonding electrons of an adjacent substituent
Describe the conformation butane.
- Rotation can occur around all three C-C bonds
- Staggered conformers of equal energy
- Eclipsed conformers of equal, but higher, energy
Why is the gauche conformation less stable than the anti conformation in butane?
Due to repulsive van der Waals forces between the two methyls
Describe the conformation of cycloalkanes.
- Molecules are not static - rotation around each C-C single bonds occurs continuously
- In rings rotation is limited - molecules twist and bend to minimise the sources of strain
What are the three sources of strain in the conformation of cycloalkanes?
- Angle strain: deviation from ideal tetrahedral bond angle
- Torsional strain: repulsion of proximate bonding electrons
- Steric strain - where atoms/groups approach too closely
