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Chemistry > 7 Organic Reaction Mechanisms > Flashcards

7 Organic Reaction Mechanisms Flashcards

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1
Q

What needs to be specified in an organic biochemical reaction?

A

substrate/starting material
reagent
product

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2
Q 
In the following reaction, state the:
                           Br₂
Cyclohexene     →      1,2 - Dibromo cyclohexane
                       hV or Δ
                          CCl₄
a) reagent
b) solvent (for alkene to dissolve in to react)
c) reaction conditions
A

a) Br₂
b) CCl₄
c) hV = light
Δ = heat

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3
Q

What two things must you look at to identify the type of reaction that took place?

A

what bonds change

what groups move

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4
Q

What do the steps of an organic reaction mechanism show and what does it lead to?

A

bond breaking and bond making (movement of electrons)

leads to product formation

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5
Q

What does a single-step organic reaction mechanism mean?

A

it is a kinetically straightforward reaction

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6
Q

What does a multiple-step organic reaction mechanism mean?

A

it involves transitory intermediate(s)

the slowest step controls reaction kinetics and determines the rate equation (R.D.S)

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7
Q

What does the overall reaction of an organic reaction mechanism determine?

A

The thermodynamics of the reaction

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8
Q

State the general structure of a 2-step organic reaction mechanism.

A

substrate (or starting material) → intermediate (measurable unstable lifetime) → product

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9
Q

On the following (see document) energy profile diagram fill in labels 1-5 as:

a) ΔHr
b) First activation energy
c) Second activation energy
d) First transition state
e) Second transition state
f) Intermediate

A 
1 - second activation energy
2 - first activation energy
3 - first transition state 
4 -  second transition state 
5 - ΔHr
6 - intermediate
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10
Q

What is bond-breaking by homolysis.

A

electrons equally divided between the 2 atoms in the bond

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11
Q

Where is homolysis common?

A

in non-polar bonds

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12
Q

What does homolysis produce?

A

radicals (X*)

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13
Q

How do radical reactions occur?

A

in distinct stages

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14
Q

Draw out the mechanism for homolysis from a C-Z bond (include single-headed ‘fish-hook’ arrows).

A

see document

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15
Q

What conditions are required for CH₄ and Br₂ to react?

A

(UV) light

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16
Q

Write out the initiation reaction between CH₄ and Br₂

A

Br₂ → Br* + Br*

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17
Q

Write out and explain the propagation reaction between CH₄ and Br₂

A

radicals react with substrate
new radicals are produced - reaction continues
CH₄ + Br* → HBr + H₃C*
Br₂ + H₃C* → CH₃Br + Br*

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18
Q

Write out and explain the termination reaction between CH₄ and Br₂

A

2 radicals combine to terminate the reaction
Br* + Br* → Br₂
Br* + H₃C* → CH₃Br
H₃C* + H₃C* → CH₃-CH₃

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19
Q

What happens at the very end of the reaction between CH₄ and Br₂?

A

bond formation between radicals

each donate one electron to form a 2-electron bond

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20
Q

Draw out the free radical use in prostaglandin and fatty acid biosynthesis in the body, naming and drawing both of the compounds.

(first compound = Hint: caterpillar head and then another and few more lines and carboxyl)

(PGH2 = Hint: tilted wizard head sneezing with headphones and poop and carboxyl)

A

see document for diagrams

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21
Q

Give 2 examples of free radical formation in the body that can cause DNA damage.

A

HO radicals from H₂O₂ (metabolism) or H₂O (irradiation)

Antioxidants form unreactive radicals i.e. vitamin E (see document for diagrams)

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22
Q

What does a polar/ionic mechanism involve?

A

the attack of electrophilic or nucleophilic reagents on polar substrate molecules

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23
Q

How do valence electrons move and where is this most common?

A

valence electrons move in pairs

most common in the lab

24
Q

Draw out the mechanism for heterolysis from a C-X bond and a C-A bond (include double-headed arrows).

A

see document

25
Q

Define ‘electrophile.’

A

fully + or δ+

26
Q

Define ‘nucleophile.’

A

Lewis bases – pair of electrons

27
Q

Why are carbocations electrophiles?

A

because the carbon centre is electron-deficient

28
Q

Are radicals electrophiles or nucleophiles and why?

A

radicals are electron-deficient and may act as electrophiles

29
Q

Why are carboanions nucleophiles?

A

because the carbon centre is electron-rich

30
Q

Explain how bond-breaking by heterolysis occurs.

A

electrons are unequally divided between the atoms in the bond
most electronegative atom/best leaving group gets 2 electrons
produces ions

31
Q

In which type of bonds does heterolysis common?

A

in polar bonds

32
Q

State how bond formation between organic ions occurs.

A

two non-metal ions with unlike charges can come together

with the negatively-charged ion donating both electrons to form the resulting two-electron bond

33
Q

Draw a diagram of bond formation between organic ions.

A

see document

34
Q

For each of the following bonds 1-4 state how they break and why.

A

1 - homolysis (O-O is a non-polar bond)
2 - heterolysis (C-I is a non-polar bond)
3 - heterolysis (C-H is a non-polar bond)
4 - heterolysis (O-H is a polar bond)

35
Q

Define ‘addition.’

A

a reaction where a small molecule is added to the starting material

36
Q

State the general reaction for addition of general molecule X-Y to a 2,3- dimethyl but-2-ene.

A

see document for diagram

37
Q

State the bonds broken and formed in the addition in alkenes.

A 
1 x (double) π-bond broken
2 x new (single) σ-bonds formed
38
Q

State and draw an equation for an example of an addition reaction and state the roles of the molecules involved.

A 
fumaric acid → malic acid 
                   (H₂O) 
              (fumarase)           
H₂O is added 
fumarase is catalyst 
(see document for symbol equation)
39
Q

Define ‘elimination.’

A

involves the removal of atoms or groups from the 2 adjacent atoms in the substrate molecule to form multiple bonds

40
Q

State the general reaction for elimination of general molecule X-Y from 2,3- X,Y but-2-ane.

A

see document for diagram

41
Q

State and draw an equation for an example of an elimination reaction and state the roles of the molecules involved.

A 
2-phosphoglycerate → phosphophoenol pyruvate 
                   (H₂O) 
              (enolase)           
H₂O is added 
enolase is catalyst 
(see document for symbol equation)
42
Q

What is the relationship between ‘elimination’ and ‘addition’ reactions?

A

they are exact opposites
a (double) π-bond is formed in elimination reactions
a (double) π-bond is broken in addition reactions

43
Q

Define ‘oxidation.’

A

removal of H₂ (or introduction of oxygen)

44
Q

Define ‘reduction.’

A

addition of H₂

45
Q

State and draw an equation for an example of a reduction reaction and state the roles of the molecules involved.

A

pyruvate → lactate
(lactate
dehydrogenase)
lactate dehydrogenase is a catalyst

46
Q

Define ‘condensation.’

A

reaction that involves combination of two molecules with elimination of a second smaller molecule

47
Q

Mechanistically, is condensation addition or elimination?

A

often a combination

48
Q

State a category of general reactions that are an example of condensation reactions.

A

peptide formation - 2 amino acid residues join together and a molecule of water is eliminated

49
Q

Write out the general reaction for peptide formation.

A

see document for this

50
Q

Define ‘substitution.’

A

an atom or group directly attached to a carbon in the substrate molecule is replaced by another atom or group

51
Q

State and draw an equation for a general substitution reaction of Y in the place of X.

A

see document for this

52
Q

What is meant by a heteroatom?

A

An atom other than C or H in the place of them

53
Q

State and draw an equation for an example of a substitution reaction and state the roles of the molecules involved.

A

ROH is also eliminated

see document for this

54
Q

Define ‘rearrangement’ reaction.

A

where molecule undergoes reorganisation of bonds to yield an isomeric product

55
Q

What migrate in a ‘rearrangement’ reaction?

A

groups or bonds

56
Q

State and draw an equation for an example of a rearrangement reaction and state the roles of the molecules involved.

A

1-butene → 2-butene
(H⁺)
Succinyl CoA ⇌ (R-)Methylmulonyl CoA

(Methylmalonyl-CoA mutase)

see document for symbol equations

Chemistry (12 decks)

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