Molecular Structure Of Organic Compounds

Question 1

Define organic chemistry

Answer 1

The study of the structure, properties, composition, reactions, and preparation (i.e. synthesis) of chemical compounds containing carbon

Question 2

What are the most common atoms found in organic compounds?

Answer 2

Carbon, hydrogen, oxygen, nitrogen and the halides (i.e. fluorine, chlorine, bromine)

Question 3

What type of bonds usually hold organic compounds together?

Answer 3

Usually covalent, but also ionic. Generally follow the octet rule

Question 4

A carbon has what types of orbitals in its outermost shell? How many bonds does this allow it form?

Answer 4

One S and three P. Four single bonds

Question 5

An oxygen may form how many bonds?

Answer 5

Two singles or one double bond

Question 6

A hydrogen can form how many bonds?

Answer 6

Only one single bond

Question 7

How many bonds can a nitrogen atom form?

Answer 7

Three single bonds

Question 8

How many bonds are halides able to form?

Answer 8

Only one single bond

Question 9

How might you tell if two elements have similar properties? (i.e. whether they form a tetrahedral structure)

Answer 9

Whether they’re in the same group or not

Question 10

Sulphur is an important component in what amino acid?

Answer 10

Cysteine

Question 11

In organic molecules, hybrid orbitals are often formed. What are these a combination of?

Answer 11

S and P orbitals

Question 12

In a carbon atom if one S and three P orbitals are mixed, what is the result?

Answer 12

Four hybrid SP3 orbitals

Question 13

Three hybridised SP2 orbitals result from what?

Answer 13

The mixing of one S and two P orbitals

Question 14

Two hybridised SP orbitals results from what?

Answer 14

The mixing of one S and two P orbitals

Question 15

What are sigma bonds? When do they form?

Answer 15

Those in which the electron density is between the nuclei. They are symmetric about the axis, can freely rotate, cross section would be circular.
They are formed when orbitals over lap directly

Question 16

Where is the electron density in pi bonds?

Answer 16

Directly over and under the plane of the atoms

Question 17

A single bond is what type?

Answer 17

A sigma bond

Question 18

A double bond is what type? Triple?

Answer 18

A sigma and a pi. A sigma and two pi

Question 19

What affect do pi bonds have on the ability of a molecule to rotate?

Answer 19

Much more rigid than a molecule which can rotate freely around its axis

Question 20

What impact does the number of bonds have on the length?

Answer 20

As a rule, it makes them shorter

Question 21

What is BE?

Answer 21

Bond dissociation energy, the energy required to break a bond. Varies with the number of bonds

Question 22

What are the BE values of sigma and pi bonds respectively?

Answer 22

110 kcal/mole.

60 kcal/mole

Question 23

What are the two ways that the delocalisation of charges in pi bonds can be represented? Describe each

Answer 23

Molecular orbit approach. Like a graphical representation where p orbitals and pi bonds are merged into a graphic which takes up the length of molecule, shows electron density.
Resonance approach. A series of formula-type equations showing location of charge at different times, need to be taken collectively

Question 24

When does the octet rule fail?

Answer 24

In elements in the third row (Si, P, S, Cl) use d-orbitals and thus can have more than eight electrons in their outer shell

Question 25

The dipole moment is a measure of what?

Answer 25

The charge separation and thus the electronegativities of the elements that make up the bond. The larger the dipole moment, the larger the separation

Question 26

When is no dipole moment found? (2 situations)

Answer 26

• When the molecule is symmetrical

- When they have identical electronegativity

Question 27

Are polar bonds or non-polar bonds generally stronger?

Answer 27

Non-polar

Question 28

Why is it that ionic bonds can be a bit stronger than covalent bonds at times? (e.g. higher melting point)

Answer 28

Because they tend to have stronger intermolecular bonds

Question 29

What are the “basics” that can supposedly solve 90% of organic chemistry problems?

Answer 29

• Opposites attract
• Likes repel
• Electronegativity
• Polarity
• Resonance

Question 30

What are spectator ions? What is their importance?

Answer 30

Ions formed by elements in the first two groups of the periodic table (Na+, K+, Ca++) do not actively engage in reactions in organic chemistry. They simply watch the reaction and then associate with the negatively charged product.
They are the exemption to the general rule; higher charge means greater force of attraction/repulsion

Question 31

In carbon based compounds, in carbon usually more or less electronegative than what it is bonded to?

Answer 31

Less electronegative

Question 32

When do carbon-carbon bonds occur?

Answer 32

When a delta positive carbon (common) binds with a delta negative carbon (rare)

Question 33

What are the two important categories of compounds which can create a carbon-carbon bond? Why these?

Answer 33

1) Alkyl lithiums (RLi)
2) Grignard reagents (RMgBr)
Because they have a delta negative carbon

Question 34

Why might a carbon be delta negative?

Answer 34

Because it is bonded to something to the left of it on the periodic table

Question 35

What is a nucleophile? Electrophile?

Answer 35

Donates two electrons in a bond.

Receives two electrons in a bond

Question 36

What is the general trend for the strength of a base with nucleophiles?

Answer 36

The stronger the nucleophile, the stronger the base. For example:
RO- > HO- > RCOO- > ROH > H2O