CBI 3: Bimolecular Bonding

Question 1

What are structural isomers?

Answer 1

• molecules that share the same molecular formula (comprise of the same atoms), but differ in the order in which atoms are bonded together
• tend to have different chemical properties as functional groups are in a different order

Question 2

What is stereoisomerism?

Answer 2

• same molecular and order in bonding of atoms (structural formula), but different 3D arrangement in space

Question 3

What is optical isomerism?

Answer 3

• a pair of isomers (enantiomers) form two non-superimposable mirror images of each other
• the isomers have equal and opposite optical activity
• they are chiral molecules that have stereocenters (chiral centres)
• optical isomers have the same compositional/molecular formula and order of bonding of their atoms, but differ in 3D arrangement in space such that the pair are non-superimposable mirror images

Question 4

How to calculate the number of stereoisomers from the number of stereocenters?

Answer 4

• 2ⁿ​

• n = no. of stereocenters

Question 5

What is absolute configuration?

Answer 5

• the description of spatial arrange of the stereocenters of a molecule
• what they are and where they are

Question 6

What are the two ways stereoisomers can be named?

Answer 6

• R/S: using Cahn-Ingold-Prelog (CIP) Rules
• D/L: using other specific rules based on the type of molecule

Question 7

Describe the Cahn-Ingold-Prelog rules

Answer 7

Question 8

Describe the CORN rules

Answer 8

Question 9

What are diastereoisomers/diastereomers?

Answer 9

• stereoisomers that differ in the absolute configuration of one or more but not all of their stereocenters
• pairs of molecules that have multiple stereocenters, but their configuration is so that both are non-identical, non-superimposable of their mirror images
• they are not enantiomers
• Unlike enantiomers, diastereomers have different physical (and often biochemical) properties even when in a symmetrical environment.

Question 10

What are epimers?

Answer 10

• diastereomers that only differ in one stereocenter

Question 11

What are anomers?

Answer 11

• a specific type of epimeric pair that form when saccharides cyclize into rings (that forms at an anomeric carbon of a cyclic saccharide as a result of acetyl/hemiacetyl formation)

Question 12

What is geometric isomerism?

Answer 12

• geometric isomers are found when there is no free rotation (restricted rotation) around two atoms (e.g. a C=C bond)
• form two non-superimposable molecules

Question 13

What is intermolecular bonding?

Answer 13

• bonding between molecules

Question 14

What is intramolecular bonding?

Answer 14

• bonding within a molecule

Question 15

Give examples of non-covalent bonding

Answer 15

• London dispersion forces
• Permanent dipole interactions
• hydrogen bonding
• ionic interactions
• hydrophobic interactions
• steric interactions

Question 16

What is an electric dipole?

Answer 16

• the difference in how positive and negative charge is distributed

Question 17

What is the magnitude of a dipole called and how does the magnitude affect the dipoles?

Answer 17

• it is called a dipole moment
• they dictate whether dipoles are permanent or transient

Question 18

What causes polarisation in bonds?

Answer 18

• when there is a significant difference in electronegativity between bonding atoms, the bonds are polarised
• the spatial distribution of electron density is not uniform

Question 19

Describe permanent dipole interactions

Answer 19

• energetically favourable interactions between molecules with permanent dipoles where opposite charges orientate towards each other
• they are weak interactions that fall off rapidly with distance
• much weaker than covalent bonds

Question 20

Describe London dispersion forces

Answer 20

• the overall distribution of electron density at any given moment will be different from the next
• transient dipoles are formed due to uneven distribution of electrons
• London dispersion forces are a type of transient dipole-transient dipole interaction
• the distribution of electrons surrounding an atom/atoms in a molecule spontaneously fluctuates and creates a temporary dipole, hence inducing a dipole in nearby electrons
• applies to all molecules
• weakest of all intermolecular forces
• strength is determined by the size of molecule (the more electrons, the greater opportunity and degree of uneven distribution of electron density)

Question 21

Describe permanent-induced dipole interactions

Answer 21

• permanent dipoles can also induce transient dipoles in neighbouring molecules
• they are intermediate in strength between permanent dipole-permanent dipole interactions and London forces

Question 22

Describe hydrogen bonding

Answer 22

• when hydrogen in a molecule is bonded to a very electronegative atom (e.g. F, O, N), the hydrogen atom can act as a ‘donor’ for a hydrogen bond with another electronegative atom (F, O, N)
• it is a type of strong dipole-dipole interaction
• not as strong as a covalent bond

Question 23

Describe ionic interactions

Answer 23

• electrostatic attraction between positively-charged and negatively-charged functional groups
• important as it impacts shape and location of many biomolecules

Question 24

What is electronegativity?

Answer 24

• The description of an intrinsic property power of an atom to attract electrons to itself

Question 25

Definition of stereocenter

Answer 25

• a geometric point in space around which chemical groups are attached, and switching the position of two of these groups give rise to another stereoisomer

Question 26

What is a dipole?

Answer 26

• A difference in how positive and negative charge is distributed in space (in a molecule); a separation of charge