Module 3: Structure And Reactions Of Organice Molecules Flashcards
(118 cards)
1
Q
Orbital
A
An area surrounding a nucleus in which an electron has a 95% probability of being within
2
Q
Order of orbitals
A
1s 2s 2p 3s 3p 4s 3d 4p 5s
3
Q
How many electrons does an S orbital contain?
A
2
4
Q
How many electrons does a p orbital contain?
A
6
5
Q
How many electrons does a d orbital contain?
A
10
6
Q
Which side of the periodic table contains elements with outermost orbitals of s?
A
Left
7
Q
Which side of the periodic table contains atoms with outermost orbitals of p?
A
Right
8
Q
How do atoms interact to form a molecule?
A
Via their outermost orbitals
9
Q
Chemical bond
A
A region of high electron density
Where electrons are repelling each other, and nuclei and electrons and attracting
10
Q
Octet rule
A
Atoms try to complete their octets (8 valence electrons) by sharing electrons
11
Q
Which row of the periodic table always obeys the octet rule?
A
Row 2
12
Q
Which row of the periodic table can disobey the octet rule?
A
Row 3
13
Q
If an atom has an excessive d orbital, which part of the periodic table is it likely from?
A
Row 3
14
Q
Five steps for drawing a Lewis structure
A
- Count valence electrons for each atom
- Assemble bonding framework using single covalent bonds
- Place three nonbonding pairs of electrons on each outer atom
- Assign remaining valence electrons to inner atoms
- Minimise formal charges on all atoms
15
Q
Formal charge= ?
A
Valence electrons on free atom - electrons assigned in Lewis structure
16
Q
How do we minimise formal charges?
A
By converting lone pairs into shared pairs (double bond)
17
Q
Resonance hybrid
A
A structure that describes chemical bonding in a molecule where there are multiple Lewis structure possibilities
18
Q
Resonance hybrid
A
The representation of two or more resonance structures of a molecule.
19
Q
What does resonance provide in organic molecules?
A
Stability
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
21
Q
Shape and bond angles of molecule with two regions of electron density
A
Linear
180 degrees
22
Q
Shape and bond angles of molecule with three regions of electron density
A
Trigonal planar
120 degrees
23
Q
Shape and bond angles of molecule with four regions of electron density
A
Tetrahedral
109 degrees
24
Q
Shape and bond angles of molecule with five regions of electron density
A
Trigonal bipyramidal
120 and 90 degrees
25
Shape and bond angles of molecule with six regions of electron density
Octahedral
| 90 degrees
26
Which element (not in the 3rd row) does not obey the octet rule?
Boron
27
Why are bond angles sometimes slightly different than normal shape would predict?
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
28
Valence bond theory
Half-filled atomic orbitals overlap to form new orbitals in a bond between atoms.
29
σ (bond)
Sigma bond (for molecular orbitals)
30
Hybrid orbital
A new orbital formed from mixing two other orbitals that is suitable for bonding.
31
How does the energy level of a hybrid orbital relate to the energy levels of the atomic orbitals?
In the middle of them
Hybrid orbitals of one molecule have the same energy as each other
32
sp^3 hybridisation
Hybrid orbitals used in any C molecule with a tetrahedral shape
33
sp^2 hybridisation
Hybrid orbitals used in any C molecule with a trigonal planar shape
34
What does a line with two arrows through it mean in an energy diagram?
A full orbital- lone pair of electrons (non-bonding)
35
Why is boron so reactive?
It has an empty p orbital which is highly attractive to electrons
36
Multiple bonding
When more than two electrons are involved in bonding two atoms
37
How many electrons involved in a double bond?
4
38
How many electrons involved in a triple bond?
6
39
If the bond angles around a central atom are 120 degrees, how many hybrid orbitals do we need to form?
Three
40
If the bond angles around a central atom are 105 degrees, how many hybrid orbitals do we need to form?
Four
41
How is a double bond formed via orbitals?
Two p orbitals form a Pi bond while two ps^2 orbitals form a sigma bond
42
You can’t have a Pi bond without _____ because?
Sigma bonds, because they pull the p orbitals together
43
Triple bonding involves____?
Two pi bonds and a sigma bond
44
Chiral organic compound
Contains an asymmetric carbon (or >1)
45
Stereocentre
An asymmetric carbon of a molecule
46
What shape will a chiral molecule be?
Tetrahedral with sp^3 orbitals
47
How do enantiomers of a compound differ?
In their interactions with other chiral compounds
48
What makes a stereocentre asymmetric?
The groups attached to it are different
49
Optical rotation
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
Which three things are displayed when describing optical rotation?
Degrees of rotation
Temperature
Light used
51
+ and - enantiomers
+ when light is rotated to the right
| - when light is rotated to the left
52
What is the result of a chiral compound containing a 1:1 of enantiomers in optical rotation?
What is this mixture called?
No rotation of light
Racemic mixture/ racemate
53
A solution containing one enantiomer is called?
Enantiopure
54
How do we identify R and S isomers? 4 steps
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
Relationship between R and S, and + and - enantiomers
Don’t always match up/ unrelated
56
Diastereomers
Any stereoisomer that isn’t an enantiomer
It has more than one stereocentre
57
How many diastereomers will a molecule with n stereocentres have?
2^n
58
What does a half-headed arrow represent?
One electron transferred
59
What does a full-headed arrow represent?
Two electrons are transferred
60
What does the direction of a half or full-headed arrow show?
Transfer of electrons to a delta positive area/ proton
61
Homolytic bond cleavage
Bond is broken by each of two electrons being transferred to a different atom
62
What are the species generated from a homolytic bond cleavage?
Radicals
63
In a double bond, which bond is cleaved first?
Pi bond
64
How is an alkene isomerised?
Pi bond is cleaved, rotation about the bond occurs, then bond forms again
65
Heterolytic bond cleavage
Both electrons are transferred from the bond to the same atom
66
What does heterolytic bond cleavage generate?
Charged intermediates
67
Electrophilic
Seeking electrons
68
Nucleophilic
Can donate a pair of electrons to form a bond
69
Leaving group
Group/ atom which separates from a molecule by breaking the bond
70
Nucleophilic substitution reaction
Leaving group (X) is replaced by nucleophile
71
Nucleophilic elimination reaction
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
Production of adrenaline is what kind of reaction?
Nucleophilic substitution
73
SN1
Substitution nucleophilic first order
Two step reaction
1. Leaving group leaves (X)
2. Nucleophile attaches
74
SN2
Substitution nucleophilic second order reaction
One step reaction: X leaves as nucleophile attaches
75
Rate law for SN1
rate = k[C-X]
76
Rate law for SN2
Rate = k[C-X][Nu:-]
77
For SN2, does it matter which side the nucleophile attaches from?
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
Does SN2 have any reaction intermediates or transition states?
One transition state: C-X bond is breaking, Nu-C bond is forming
79
Which have higher Gibbs energy in a nucleophilic reaction- products or reactants?
Reactants
80
Which step is slower in an SN1 reaction?
First step- X leaves slowly
Is independent of nucleophile
81
First transition state in SN1 reaction
C-X bond is stretched to breaking point
| nucleophile is not yet attached
82
Why is the second step of SN1 fast?
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
Reaction intermediate of SN1 reaction
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
Second transition state of SN1 reaction
C-Nu bond is forming
85
When will SN1 be a three step process? Why?
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
Inversion of configuration
R to S or S to R
87
What happens to stereochemistry in SN2 mechanism?
Nucleophile attaches opposite to the leaving group, so stereochemistry is inverted
88
Which nucleophilic substitution is stereospecific?
SN2
89
Why does SN1 mechanism produce both R and S enantiomers?
Because the nucleophile attaches to the carbocation (no X group), so it can approach from either side
90
Which kind of carbon molecules almost always goes via SN2?
Methyl
91
Which kind of carbon molecule almost always goes via SN1?
Tertiary (3 R side chains/ 3 additional carbons)
92
Why is SN1 faster with a higher number of alkyl groups?
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
Why is SN2 faster with a lower number of alkyl groups?
The less alkyl groups, the less steric hinderance of the nucleophile trying to approach the carbon
94
What two components of a secondary carbon reaction determine whether it goes via SN1 or SN2?
Leaving group X
| Solvent
95
What does SN1 require that SN2 doesn’t? Why?
A good leaving group
In SN2, strong nucleophile can make up for a poor leaving group because it’s done in one step
96
What makes a good leaving group? Three examples
A weak base (conjugate acid is strong)
Cl-
Br-
I-
97
Which of I:-, Br:- and Cl:- is the best leaving group? Why?
I:- because it has the fastest rate of C-X cleavage, due to its longer weaker bonds (bigger atomic number)
98
Example of bad leaving group
| Why?
HO:-
Strong base forms strong bond with carbon. Slow rate of C-X cleavage
99
How can HO:- or RO:- be converted into better leaving groups?
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
In which substitution reaction is a good nucleophile essential? Why?
SN2
Has to bump electrophile off the carbon
101
Example of a good nucleophile, and a neutral nucleophile
Good: HO:-
Neutral: H2Ö
102
Why is (CH3)3 CO:- a poor nucleophile?
Three methyl groups gives molecule a large size - they don’t allow the O:- to get close enough to the electrophilic carbon
103
In which solvents is SN1 fastest? Why?
Polar solvents, particularly those which hydrogen-bond
First transition state is stabilised by hydrogen bonds- making bond cleavage (first step) faster and easier
104
Which solvents are best for SN2 reactions? Why?
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
What are elimination mechanisms determined by?
Base strength
Strong base: E2
Weak base: E1
106
Describe E1 mechanism
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
Controlling factors of E1 reaction (same as SN1) (3)
Strong base not required
H-bonding polar solvents speed up reaction
Most common when leaving group is on tertiary carbon
108
Which two nucleophilic reactions often co-occur? What is the ratio of these two reactions controlled by?
E1 and SN1
Relative rates of the fast step of each reaction (does not depend on leaving group)
109
Describe E2 mechanism
Nucleophile pulls proton off and X leaves (forming alkene bond) in same step
Second order reaction
110
Where does the nucleophile collide in E2 reaction?
At the carbon peripheral to the leaving group
111
Does E2 need a strong base, or does it not matter?
Yes- protons aren’t as easy to pull off since X is still attached
112
Why is E2 possible for 1, 2 and 3 carbon compounds?
The protons are easily accessible in any compound- on the outside
113
What does ENu represent?
A molecule with an electrophilic portion and a nucleophilic portion
114
Two steps of addition reaction to an alkene
1. Slow addition of the electrophilic to the double bond
| 2. Fast attack by a nucleophile at the carbocation intermediate
115
Which part of an addition to alkene requires the highest energy?
The first step- slow and rate-determining
116
Which carbon of unsymmetrical carbon does the nucleophile attack in an alkene addition reaction? Why?
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
How can a non polar molecule (e.g. Br2) react with an alkene by addition?
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
How is the carbocation stabilised in addition to an alkene?
Br (electrophilic) snaps shut by forming single bond with each carbon (two)
This forms a bromonium ion for example
