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MCAT Organic Chemistry > Molecular Structures > Flashcards

Molecular Structures Flashcards

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1
Q

What is the octet rule in organic chemistry?

A

Most ionic/covalent bonds in organic chemistry bond in such a way that they can achieve the electron configuration of the nearest noble gas, usually eight electrons.

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2
Q

Describe the bonding ability of carbon

A

A carbon atom has one s and three p orbitals in its outermost shell, allowing it to form 4 single bonds, 2 double bonds, or a triple bond.

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3
Q

Describe the bonding ability of oxygen

A

An oxygen atom may form 2 single bonds or one double bond.

It has 2 unshared (lone) electron pairs

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4
Q

Describe the bonding ability of hydrogen

A

A hydrogen atom can only form one single bond

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5
Q

Describe the bonding ability of nitrogen

A

A nitrogen atom may form 3 single bonds, or double and triple bonds.

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6
Q

Describe the bonding ability of the halides (eg. F, Cl, Br etc.)

A

The halides are all able to form only one single bond,

They have three unshared electron pairs

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7
Q

How many electrons do(es) hydrogen, oxygen, nitrogen and carbon need in their outer shell to become stable?

A

HONC!!!

H: 1
O: 2
N: 3
C: 4

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8
Q

What are hybrid orbitals?

A

Orbitals of atoms are combined in organic molecules to form hybrid orbitals, consisting of a mixture of s and p orbitals.

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9
Q

What is the geometry of a sp3 hybridized orbital?

A

tetrahedral

109.5 angle between orbitals

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10
Q

What is the geometry of a sp2 hybridized orbital?

A

Triangular

120 degree angle between orbitals

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11
Q

What is the geometry of a sp hybridized orbital?

A

linear (180 degree angle)

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12
Q

What are sigma bonds in organic molecules? (5)

A
  • Single bonds in which the electron density is between the nuclei.
  • They are symmetrical about the axis
  • They can freely rotate
  • Formed when orbitals (regular or hybridized) overlap directly
  • Characterized by the fact that they are circular when a cross section is taken and the bond is viewed along the bond axis
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13
Q

What are pi bonds in organic molecules?

What are double and triple bonds composed of?

A
  • Exist when double (or higher) bonds are formed
  • The electron density in pi bonds overlaps both above and below the plane of the atoms.
  • Double bonds are 1 σ bond + 1 π bond
  • Triple bonds are 1 σ bond + 2 π bonds
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14
Q

When two sp2 hybridized atoms form a bond, which orbitals become pi bonds and which become sigma bonds?

A

pi bonds: p orbitals overlap above and below the axis

sigma bonds: sp2 hybrids overlap between nuclei to form sigma bonds

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15
Q

What effects do multiple bonds have on molecules?

A
  • pi bonds create a barrier to free rotation about the axis of the bond (multiply bonded molecules are much more ‘rigid’ because of this)
  • The length of a bond decreases with multiple bonds
  • Bond dissociation energy (energy to break a bond) increases with multiple bonds
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16
Q

When is delocalization of charges in pi bonds possible? And what does this result in?

A

Possible when there are hybridized orbitals in adjacent atoms. This delocalization is represented by resonance structures and a resonance hybrid.

17
Q

List the most electronegative elements in order with their electronegativities.

What is a consequence of binding with these elements?

A

Fluorine (4)
Oxygen (3.5)
Nitrogen (3)
Chlorine (3)

These are often bonded with carbon (2.5) and hydrogen (2.1), resulting in partially ionic bonds and dipole moments, these molecules are polar.

Dipole moments are not found when the charge separation is symmetric (because they are cancelled out like opposing vectors), these molecules are not polar.

18
Q

Describe the strength difference between polar and non-polar bonds

A 
Strongest
 - Covalent
 - Polar-covalent
 - Ionic 
 - Hydrogen bonds
Weakest

Non-polar bonds are generally stronger than polar covalent and ionic bonds, with ionic bonds being the weakest.

However, large molecules with lots of ionic bonds are very strong.

19
Q

How do ions formed by elements in the first two groups of the periodic table react in organic reactions (eg. Na+, K+, Ca++)? What are these ions called?

A

These do not activity engage in the reactions in organic chemistry. They associated with the negatively charged product at the very end.

For this reason they are called spectator ions.

20
Q

How can compounds create carbon-carbon bonds?

What are two important categories of compounds that can do this?

A

Because opposites attract, a δ- carbon (which is unusual) would create a carbon-carbon bond with a δ+ carbon (which is common).

Two important categories of compounds can create c-c bonds with δ- carbons:

  • Alkyl lithiums (R-Li)
  • Grignard reagents (R-MgBr)
21
Q

What is the general trend with nucleophiles and basicity/acidity?

A

The stronger the nucleophile, the stronger the base.

22
Q

Define Markovnikov’s rule

A

The chemical basis for Markovnikov’s Rule is the formation of the most stable carbocation during the addition process. The addition of the hydrogen ion to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate.

The rule states that with the addition of a protic acid HX to an unsymmetrical alkene, the acid hydrogen (H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents.

23
Q

What is an anti-Markovnikov addition?

A

Mechanisms that avoid the carbocation intermediate may react through other mechanisms that are regioselective, against what Markovnikov’s rule predicts, such as free radical addition.

Such reactions are said to be anti-Markovnikov, since the halogen adds to the LESS substituted carbon, exactly the opposite of Markovnikov reaction.

The radical is most stable when in the more substituted position.

24
Q

According to IUPAC, how must you name a cyclooctane with three different (but saturated) alkyl groups?

A

Alkyl groups do not give order based on number of carbons, so they are put in the name alphabetically. The numbering of carbons must be so that the total numbers are the lowest possible.

Eg. 3-ethyl-1-methyl-5-propylcyclooctane is correct

1-ethyl-3-methyl-7-propylcyclooctane is not correct (numbers add higher than they could).

25
Q

Below is a list of 4 cyclohexanes, each with two substituents. Predict which substituents should be axial/equatorial for the lowest energy conformation.

A. isopropyl, tertiary butyl
B. amino group, methyl group
C. bromide group, methyl group
D. amino group, hydroxyl group

A

A. Axial: isopropyl, equatorial: tert butyl

B. Axial: amino group, equatorial: methyl group

C. Axial: bromide group, equatorial: methyl group

D. Axial: hydroxyl group, equatorial amino group

ΔE: tert butyl > isopropyl > methyl > amino > hydroxyl

26
Q

What is Hückel’s Rule?

A

Hückel’s Rule (4n+2 rule): In order to be aromatic, a molecule must have a certain number of pi electrons (electrons with pi bonds or lone pairs) within a closed loop of parallel, adjacent p orbitals. The pi electron count is defined by the series 4n+2 where n = zero or a positive integer (0, 1, 2, etc.). The most common case in six pi electrons (n = 1) which is found for example in benzene, pyrrole, furan, and pyridine.

27
Q

What bond angles do 3 member rings have?

A

60 degrees.

This is because they are triangles and must have angles that sum to 180 degrees.

28
Q

Match the following:

A methoxide will show selectivity for the carbon atom that has the greatest carbocation character in:

SN2 or E2?

A methoxide will show selectivity for the carbon atom that has the least steric hindrance in

SN2 or E2

A

Match the following:

A methoxide will show selectivity for the carbon atom that has the greatest carbocation character in:

E2

A methoxide will show selectivity for the carbon atom that has the least steric hindrance in

SN2 (always)

29
Q

What effect does resonance stabilization have on acidic protons?

A

Greater resonance stabilization will increase the acidity of the proton

The reactivity of the alpha-hydrogen arises from resonance stabilization of the conjugate base after removal of the hydrogen

MCAT Organic Chemistry (21 decks)

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