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CHEM > Module 3: Structure And Reactions Of Organice Molecules > Flashcards

Module 3: Structure And Reactions Of Organice Molecules Flashcards

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1
Q

Orbital

A

An area surrounding a nucleus in which an electron has a 95% probability of being within

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2
Q

Order of orbitals

A 
1s
2s
2p
3s
3p
4s
3d
4p
5s
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3
Q

How many electrons does an S orbital contain?

A

2

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4
Q

How many electrons does a p orbital contain?

A

6

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5
Q

How many electrons does a d orbital contain?

A

10

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6
Q

Which side of the periodic table contains elements with outermost orbitals of s?

A

Left

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7
Q

Which side of the periodic table contains atoms with outermost orbitals of p?

A

Right

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8
Q

How do atoms interact to form a molecule?

A

Via their outermost orbitals

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9
Q

Chemical bond

A

A region of high electron density

Where electrons are repelling each other, and nuclei and electrons and attracting

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10
Q

Octet rule

A

Atoms try to complete their octets (8 valence electrons) by sharing electrons

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11
Q

Which row of the periodic table always obeys the octet rule?

A

Row 2

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12
Q

Which row of the periodic table can disobey the octet rule?

A

Row 3

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13
Q

If an atom has an excessive d orbital, which part of the periodic table is it likely from?

A

Row 3

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14
Q

Five steps for drawing a Lewis structure

A
  1. Count valence electrons for each atom
  2. Assemble bonding framework using single covalent bonds
  3. Place three nonbonding pairs of electrons on each outer atom
  4. Assign remaining valence electrons to inner atoms
  5. Minimise formal charges on all atoms
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15
Q

Formal charge= ?

A

Valence electrons on free atom - electrons assigned in Lewis structure

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16
Q

How do we minimise formal charges?

A

By converting lone pairs into shared pairs (double bond)

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17
Q

Resonance hybrid

A

A structure that describes chemical bonding in a molecule where there are multiple Lewis structure possibilities

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18
Q

Resonance hybrid

A

The representation of two or more resonance structures of a molecule.

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19
Q

What does resonance provide in organic molecules?

A

Stability

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20
Q

VSEPR

A

Valence shell electron pair repulsion

A theory that states a molecule has the shape which allows pairs to be as far away form each other as possible

Or electron pair repulsions are minimised

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21
Q

Shape and bond angles of molecule with two regions of electron density

A

Linear

180 degrees

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22
Q

Shape and bond angles of molecule with three regions of electron density

A

Trigonal planar

120 degrees

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23
Q

Shape and bond angles of molecule with four regions of electron density

A

Tetrahedral

109 degrees

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24
Q

Shape and bond angles of molecule with five regions of electron density

A

Trigonal bipyramidal

120 and 90 degrees

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25
Q

Shape and bond angles of molecule with six regions of electron density

A

Octahedral

90 degrees

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26
Q

Which element (not in the 3rd row) does not obey the octet rule?

A

Boron

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27
Q

Why are bond angles sometimes slightly different than normal shape would predict?

A

Lone electron pairs have bigger regions of electron density than bonding pairs

This increases the bond angles between lone pairs and bonding pairs

And decreases the bond angles between separate bonding pairs

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28
Q

Valence bond theory

A

Half-filled atomic orbitals overlap to form new orbitals in a bond between atoms.

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29
Q

σ (bond)

A

Sigma bond (for molecular orbitals)

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30
Q

Hybrid orbital

A

A new orbital formed from mixing two other orbitals that is suitable for bonding.

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31
Q

How does the energy level of a hybrid orbital relate to the energy levels of the atomic orbitals?

A

In the middle of them

Hybrid orbitals of one molecule have the same energy as each other

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32
Q

sp^3 hybridisation

A

Hybrid orbitals used in any C molecule with a tetrahedral shape

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33
Q

sp^2 hybridisation

A

Hybrid orbitals used in any C molecule with a trigonal planar shape

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34
Q

What does a line with two arrows through it mean in an energy diagram?

A

A full orbital- lone pair of electrons (non-bonding)

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35
Q

Why is boron so reactive?

A

It has an empty p orbital which is highly attractive to electrons

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36
Q

Multiple bonding

A

When more than two electrons are involved in bonding two atoms

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37
Q

How many electrons involved in a double bond?

A

4

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38
Q

How many electrons involved in a triple bond?

A

6

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39
Q

If the bond angles around a central atom are 120 degrees, how many hybrid orbitals do we need to form?

A

Three

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40
Q

If the bond angles around a central atom are 105 degrees, how many hybrid orbitals do we need to form?

A

Four

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41
Q

How is a double bond formed via orbitals?

A

Two p orbitals form a Pi bond while two ps^2 orbitals form a sigma bond

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42
Q

You can’t have a Pi bond without _____ because?

A

Sigma bonds, because they pull the p orbitals together

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43
Q

Triple bonding involves____?

A

Two pi bonds and a sigma bond

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44
Q

Chiral organic compound

A

Contains an asymmetric carbon (or >1)

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45
Q

Stereocentre

A

An asymmetric carbon of a molecule

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46
Q

What shape will a chiral molecule be?

A

Tetrahedral with sp^3 orbitals

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47
Q

How do enantiomers of a compound differ?

A

In their interactions with other chiral compounds

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48
Q

What makes a stereocentre asymmetric?

A

The groups attached to it are different

49
Q

Optical rotation

A

Polarises and rotates light by cutting out specific wavelengths

Passes the light through a chiral compound in solution

If the light rotates 90 degrees, the molecule is an enantiomer

50
Q

Which three things are displayed when describing optical rotation?

A

Degrees of rotation
Temperature
Light used

51
Q

+ and - enantiomers

A

+ when light is rotated to the right

- when light is rotated to the left

52
Q

What is the result of a chiral compound containing a 1:1 of enantiomers in optical rotation?

What is this mixture called?

A

No rotation of light

Racemic mixture/ racemate

53
Q

A solution containing one enantiomer is called?

A

Enantiopure

54
Q

How do we identify R and S isomers? 4 steps

A
  1. Identify stereocentre
  2. Assign priorities to the four substituents
  3. Visualise molecule with lowest priority substituent in the back
  4. R: 1-3 bond arrangement is clockwise, L: 1-3 bond arrangement is anticlockwise
55
Q

Relationship between R and S, and + and - enantiomers

A

Don’t always match up/ unrelated

56
Q

Diastereomers

A

Any stereoisomer that isn’t an enantiomer

It has more than one stereocentre

57
Q

How many diastereomers will a molecule with n stereocentres have?

A

2^n

58
Q

What does a half-headed arrow represent?

A

One electron transferred

59
Q

What does a full-headed arrow represent?

A

Two electrons are transferred

60
Q

What does the direction of a half or full-headed arrow show?

A

Transfer of electrons to a delta positive area/ proton

61
Q

Homolytic bond cleavage

A

Bond is broken by each of two electrons being transferred to a different atom

62
Q

What are the species generated from a homolytic bond cleavage?

A

Radicals

63
Q

In a double bond, which bond is cleaved first?

A

Pi bond

64
Q

How is an alkene isomerised?

A

Pi bond is cleaved, rotation about the bond occurs, then bond forms again

65
Q

Heterolytic bond cleavage

A

Both electrons are transferred from the bond to the same atom

66
Q

What does heterolytic bond cleavage generate?

A

Charged intermediates

67
Q

Electrophilic

A

Seeking electrons

68
Q

Nucleophilic

A

Can donate a pair of electrons to form a bond

69
Q

Leaving group

A

Group/ atom which separates from a molecule by breaking the bond

70
Q

Nucleophilic substitution reaction

A

Leaving group (X) is replaced by nucleophile

71
Q

Nucleophilic elimination reaction

A

Nucleophile acts as a base and pulls hydrogen atom from carbon (peripheral to leaving group)

Leaving group’s bond is broken and electrons from that bond form a double bond, turning alkane into an alkene

72
Q

Production of adrenaline is what kind of reaction?

A

Nucleophilic substitution

73
Q

SN1

A

Substitution nucleophilic first order

Two step reaction

  1. Leaving group leaves (X)
  2. Nucleophile attaches
74
Q

SN2

A

Substitution nucleophilic second order reaction

One step reaction: X leaves as nucleophile attaches

75
Q

Rate law for SN1

A

rate = k[C-X]

76
Q

Rate law for SN2

A

Rate = k[C-X][Nu:-]

77
Q

For SN2, does it matter which side the nucleophile attaches from?

A

Yes- opposite to leaving group, as needs to happen simultaneously

(if it approaches from X side, X acts as a barrier and the nucleophile can’t get enough energy)

78
Q

Does SN2 have any reaction intermediates or transition states?

A

One transition state: C-X bond is breaking, Nu-C bond is forming

79
Q

Which have higher Gibbs energy in a nucleophilic reaction- products or reactants?

A

Reactants

80
Q

Which step is slower in an SN1 reaction?

A

First step- X leaves slowly

Is independent of nucleophile

81
Q

First transition state in SN1 reaction

A

C-X bond is stretched to breaking point

nucleophile is not yet attached

82
Q

Why is the second step of SN1 fast?

A

The carbon is now a carbocation, because the negative X group has left

This means it is charged and accessible, so the nucleophile doesn’t have to collide with a particular orientation

83
Q

Reaction intermediate of SN1 reaction

A

Carbocation (positive carbon group with no X or nucleophile attaches)

Reacts with any nucleophile it collided with- doesn’t have to be a good one

Has a short lifetime

84
Q

Second transition state of SN1 reaction

A

C-Nu bond is forming

85
Q

When will SN1 be a three step process? Why?

A

When H2O is the solvent (anything aqueous)

H2O provides one extra proton (H atom) to the carbocation, so it has a positive charge

the third step involves deprotonation by a base (can be H2O)

86
Q

Inversion of configuration

A

R to S or S to R

87
Q

What happens to stereochemistry in SN2 mechanism?

A

Nucleophile attaches opposite to the leaving group, so stereochemistry is inverted

88
Q

Which nucleophilic substitution is stereospecific?

A

SN2

89
Q

Why does SN1 mechanism produce both R and S enantiomers?

A

Because the nucleophile attaches to the carbocation (no X group), so it can approach from either side

90
Q

Which kind of carbon molecules almost always goes via SN2?

A

Methyl

91
Q

Which kind of carbon molecule almost always goes via SN1?

A

Tertiary (3 R side chains/ 3 additional carbons)

92
Q

Why is SN1 faster with a higher number of alkyl groups?

A

First transition state (C-X bond breaking) can be stabilised by electrons from alkyl groups

The more alkyl groups, the more they can stabilise

93
Q

Why is SN2 faster with a lower number of alkyl groups?

A

The less alkyl groups, the less steric hinderance of the nucleophile trying to approach the carbon

94
Q

What two components of a secondary carbon reaction determine whether it goes via SN1 or SN2?

A

Leaving group X

Solvent

95
Q

What does SN1 require that SN2 doesn’t? Why?

A

A good leaving group

In SN2, strong nucleophile can make up for a poor leaving group because it’s done in one step

96
Q

What makes a good leaving group? Three examples

A

A weak base (conjugate acid is strong)

Cl-
Br-
I-

97
Q

Which of I:-, Br:- and Cl:- is the best leaving group? Why?

A

I:- because it has the fastest rate of C-X cleavage, due to its longer weaker bonds (bigger atomic number)

98
Q

Example of bad leaving group

Why?

A

HO:-

Strong base forms strong bond with carbon. Slow rate of C-X cleavage

99
Q

How can HO:- or RO:- be converted into better leaving groups?

A

Protonation under strongly acidic conditions

O bonds a second H atom, making the leaving group H2O

A much better leaving group with weaker C-O bond- because electrons move closer and to O from C

100
Q

In which substitution reaction is a good nucleophile essential? Why?

A

SN2

Has to bump electrophile off the carbon

101
Q

Example of a good nucleophile, and a neutral nucleophile

A

Good: HO:-
Neutral: H2Ö

102
Q

Why is (CH3)3 CO:- a poor nucleophile?

A

Three methyl groups gives molecule a large size - they don’t allow the O:- to get close enough to the electrophilic carbon

103
Q

In which solvents is SN1 fastest? Why?

A

Polar solvents, particularly those which hydrogen-bond

First transition state is stabilised by hydrogen bonds- making bond cleavage (first step) faster and easier

104
Q

Which solvents are best for SN2 reactions? Why?

A

Polar solvents that don’t do hydrogen bonding

Solvent sphere (circle of H+ bonds) can hinder the nucleophile- and hinder the process of nucleophile bonding to the carbon

105
Q

What are elimination mechanisms determined by?

A

Base strength

Strong base: E2
Weak base: E1

106
Q

Describe E1 mechanism

A

Step 1: C-X bond cleaved, forming carbocation

Step 2: nucleophile bonds to H atom and pulls it from the carbocation. Electrons from C-H bond form a Pi bond (double bond) to the central carbon- forming an alkene

107
Q

Controlling factors of E1 reaction (same as SN1) (3)

A

Strong base not required
H-bonding polar solvents speed up reaction
Most common when leaving group is on tertiary carbon

108
Q

Which two nucleophilic reactions often co-occur? What is the ratio of these two reactions controlled by?

A

E1 and SN1

Relative rates of the fast step of each reaction (does not depend on leaving group)

109
Q

Describe E2 mechanism

A

Nucleophile pulls proton off and X leaves (forming alkene bond) in same step

Second order reaction

110
Q

Where does the nucleophile collide in E2 reaction?

A

At the carbon peripheral to the leaving group

111
Q

Does E2 need a strong base, or does it not matter?

A

Yes- protons aren’t as easy to pull off since X is still attached

112
Q

Why is E2 possible for 1, 2 and 3 carbon compounds?

A

The protons are easily accessible in any compound- on the outside

113
Q

What does ENu represent?

A

A molecule with an electrophilic portion and a nucleophilic portion

114
Q

Two steps of addition reaction to an alkene

A
  1. Slow addition of the electrophilic to the double bond

2. Fast attack by a nucleophile at the carbocation intermediate

115
Q

Which part of an addition to alkene requires the highest energy?

A

The first step- slow and rate-determining

116
Q

Which carbon of unsymmetrical carbon does the nucleophile attack in an alkene addition reaction? Why?

A

The carbon with more alkyl groups donating electrons

The alkyl groups stabilise the positive charge of the carbocation (onto which the nucleophile attacks)

This takes less energy to form

117
Q

How can a non polar molecule (e.g. Br2) react with an alkene by addition?

A

As it approaches the double bond- which is electron rich- a dipole is induced

Turning one Br more positive and one more negative (electron dense)

118
Q

How is the carbocation stabilised in addition to an alkene?

A

Br (electrophilic) snaps shut by forming single bond with each carbon (two)

This forms a bromonium ion for example

CHEM (3 decks)

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