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Medicinal Chemistry > Molecular Shape and Reactivity > Flashcards

Molecular Shape and Reactivity Flashcards

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1
Q

What does orbital hybridisation account for?

A

The bonding of carbon atoms into chains and rings.

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2
Q

Describe the bonding in ethane.

A
  • 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
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3
Q

Describe the bonds in ethene.

A
  • 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
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4
Q

Describe how the double bond forms in ethene.

A
  • 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
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5
Q

How many electrons are involved in the formation of a C=C bond?

A

four

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6
Q

Compare a C=C bond to a C-C bond.

A

C=C bond is stronger and shorter than C-C

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7
Q

Why does the rotational barrier exist in ethene?

A

the p orbitals are well aligned for maximum overlap leading to formation of the pi bond.

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8
Q

Describe the bonds in ethyne.

A
  • 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
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9
Q

How many electrons are involved in the formation of a C C triple bond?

A

six

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10
Q

Compare a C triple bond to a C=C

A

C triple bond is stronger and shorter than a C=C bond

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11
Q

Describe the bonding in methyl cation.

A
  • 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
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12
Q

Describe the bonding in methyl radical

A
  • 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
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13
Q

Describe the bonding in methyl anion

A
  • 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
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14
Q

Describe the bonding in nitrogen

A
  • 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
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15
Q

Describe the bonding in amines

A

C-N bonds formed by sp3-sp3 overlap
- Hydrogens may be replaced by 1/2/3 alkyl or aryl groups
- Electron rich therefore nucleophilic

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16
Q

Describe the bonding in ammonium cations

A
  • Possesses four identical N-H bonds and no lone pairs
  • Nitrogen has four sp3 hybridised orbitals
  • All bond angles = 109.5
17
Q

Describe the bonding in oxygen

A
  • 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)
18
Q

Describe the bonding in sulphur atoms

A
  • 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
19
Q

Describe the bonding in hydrogen halides

A
  • 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
20
Q

Describe the bonding in Alkyl Halides

A
  • 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
21
Q

Define electronegativity.

A
  • The tendency of an atom to pull bonding electrons toward itself
22
Q

Define polar covalent bond.

A
  • A covalent bond in which the electron distribution between atoms is unsymmetrical
  • Bond polarity is due to difference in electronegativity
23
Q

Describe the inductive effect.

A
  • 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
24
Q

Define dipole moment.

A
  • A measure of the net polarity of a molecule
25
Q

What are the four main types of interactions in order of increasing strength?

A
  1. Van der Waals forces
  2. Dipole-Dipole interactions
  3. Hydrogen bonding
  4. Ion-Ion (Electrostatic)
26
Q

Describe Van der Waals forces.

A
  • 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
27
Q

Define polarisability (van der Waals)

A
  • A measure of how the electron cloud around an atom responds to changes in its electron environment
28
Q

Describe dipole-dipole interactions

A
  • Attractive forces between the permanent dipoles of two polar molecules.
29
Q

Describe hydrogen bonding.

A
  • 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
30
Q

In ethane which two extreme conformations are possible?

A

Staggered and eclipsed

31
Q

What is meant by torsional strain?

A
  • Repulsion felt by bonding electrons as they pass close to the bonding electrons of an adjacent substituent
32
Q

Describe the conformation butane.

A
  • Rotation can occur around all three C-C bonds
  • Staggered conformers of equal energy
  • Eclipsed conformers of equal, but higher, energy
33
Q

Why is the gauche conformation less stable than the anti conformation in butane?

A

Due to repulsive van der Waals forces between the two methyls

34
Q

Describe the conformation of cycloalkanes.

A
  • 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
35
Q

What are the three sources of strain in the conformation of cycloalkanes?

A
  • Angle strain: deviation from ideal tetrahedral bond angle
  • Torsional strain: repulsion of proximate bonding electrons
  • Steric strain - where atoms/groups approach too closely
36
Q
A

Medicinal Chemistry (21 decks)

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