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AS Organic CHEM > Nucleophillic Substitution > Flashcards

Nucleophillic Substitution Flashcards

1
Q

What do mechanisms show?

A
  • Mechanisms show the movement of pairs of electrons.
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2
Q

How is the formation of a covalent bond shown by mechanisms?

A
  • Formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond.

() - extra info for clarification

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3
Q

How is the breaking of a covalent bond shown by mechanisms?

A
  • Shown by a curly arrow starting from the middle of a bond (to another atom.)

() - extra info

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4
Q

Where do curly arrows always start?

A
  • Curly arrows always start from an area with electrons ie. a bond/ lone pair of electrons.
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5
Q

What is a nucleophile?

A
  • A nucleophile is a species which donates a pair of electrons to an electron-deficient centre, forming a dative covalent bond.
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6
Q

Give 3 examples of nucleophilles.

A
  • Ammonia (:NH₃)
  • Cyanide ions (:CN⁻)
  • Hydroxide ions (OH⁻) –> have two lone pairs of electrons.
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7
Q

What type of bonds do halogenoalkanes have? Explain why this is the case

A
  • Polar bonds.
  • Because halogens are more electronegative than carbon , so the halogens will draw the electrons in the covalent bond closer to themselves.
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8
Q

What allows halogenoalkanes to be able to be attacked by nucleophilles?

A
  • The POLAR BOND means that halogenoalkanes can be attacked by nucleophiles.
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9
Q

What is the reaction called when nucleophilles (OH-/ CN-/ NH3) attack halogenoalkanes ?

A
  • Nucleophillic substitution
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10
Q

What are the conditions for the reaction of nucleophillic substitution between halogenoalkanes and hydroxide ions?

A
  • Warm aqueous NaOH (source of OH- ions.) (warm will be achieved by heating reaction mixture.)
  • Carried out under reflux
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11
Q

What does “carried out under reflux” mean?

A
  • Condenser on top of test-tube so if any substance evaporated (due to the heat), it will be condensed straight back into reaction mixture.
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12
Q

Outline the mechanism of nucleophillic substitution of chloroethane using NaOH.

Diagram of this, which you need to know, is drawn on piece of paper - found in pink folder.

A
  • Nucleophile (OH- ions) will attack S+ carbon.
  • Carbon-halogen bond breaks, pair of electrons move from the bond to the halogen.
  • A new bond is formed between OH- ion and carbon.
  • A halide ion is released.
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13
Q

Overall equation for nucleophillic substitution using NaOH. NAME PRODUCTS

A

RX + OH- –> ROH + X-
(there is Na before OH on left-hand side and before Br- on right, but they are spectator ions so cancel out.)
(R = alkyl group X = halogen.)
PRODUCTS: halide ion/ alcohol

() - extra info

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14
Q

What is the role of OH-/ CN- in nucleophillic substitution reactions?

A
  • They are nucleophilles.
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15
Q

Outline the mechanism of nucleophillic substitution of chloroethane using CN- ions.

Diagram of this, which you need to know, is drawn on piece of paper - found in pink folder.

A
  • Nucleophile (CN- ions) will attack S+ carbon.
  • Carbon-halogen bond breaks, pair of electrons move from the bond to the halogen.
  • A new bond is formed between CN- ion and carbon.
  • A halide ion is released.
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16
Q

What are the conditions for nucleophillic substitution of halogenoalkane using CN- ions?

A
  • Warm ethanolic potassium cyanide (source of CN- ions.)
  • Carried out under reflux.
17
Q

Overall equation for nucleophillic substitution of halogenoalkane using CN- ions. NAME THE PRODUCTS

A

RX + CN- —> RCN + X-
( R= alkly group/ X= halogen.)
(there is K on left hand side before CN and on right before X- but they are spectator ions!!)
PRODUCTS: nitrile/ halide ion

() - extra info for clarification.

18
Q

Outline the mechanism of nucleophillic substitution of chloroethane using ammonia (FIRST STAGE!!!)

Diagram of this, which you need to know, is drawn on piece of paper - found in pink folder.

A
  • Nucleophille (ammonia) attacks S+ carbon.
  • Carbon-halogen bond breaks, pair of electrons move from the bond to the halogen.
  • New bond between NH3 and S+ carbon.
  • Intermediate is formed.
19
Q

Outline the mechanism of nucleophillic substitution of chloroethane using ammonia (SECOND STAGE!!!)

Diagram of this, which you need to know, is drawn on piece of paper - found in pink folder.

A
  • Another molecule of NH3 acts as a base and reacts with hydrogen.
  • N+ –H bond breaks, pair of electrons move from the bond to the N+, stabilising the N+ (note that N+ doesn’t become N- because technically it only GAINS 1 electron, but we say a “pair of electrons” is moving because “technically” that 1 electron that belongs to ammonia belongs to the bond, not the N directly.)

() - extra info for clarification

20
Q

Why do we call the NH3 molecule (in the second stage of nucleophillic substitution using NH3) a base?

A
  • Because bases will accept protons so will react with H+ ions.
21
Q

Conditions required for nuelcophillic substitution of halogenoalkane using NH3?

A
  • Heat with ethanolic ammonia.
  • Must have an excess of ammonia.
22
Q

Why must we have excess of ammonia in nucleophillic substitution reaction of halogenoalkane using NH3?

A
  • So that there’s enough ammonia for 2nd molecule to act as a base and react with hydrogen.
23
Q

Overall equation for nucleophillic substitution of halogenoalkane using NH3? Why aren’t the halogens treated as spectator ions (ie. why don’t they cancel each other out?)

A

RX + 2NH3 –> RNH2 + NH4X
(R = alkyl group, X = halogen.)
Halogens not treated as spectator ions as on one side they are covalently bonded, ie. on one side it’s not ion!!
PRODUCTS: Amine/ salt

24
Q

What does NH3 act as in nucleophillic substitution of halogenoalkane?

A
  • First molecule in first phase acts as nucleophille.
  • Second molecule in second phase acts as base.
25
Q

What type of reaction is nucleophillic substitution of halogenoalkane using NH3?

A
  • Reversible reaction
26
Q

After the NH3 attacks the S+ carbon, and the bond is broken between C-Cl and a new bond is formed between C and NH3, why does the N have a positive charge? What does this mean for the N?

A
  • Because, the N forms 4 bonds when N can only form 3 so, an electron from its lone pair is within a covalent bond (rather than on the N itself)- giving it an overall + charge.
  • Means N is unstable/ needs to be stabilised
27
Q

How does the bond enthalpy of the carbon-halogen bond in halogenoalkanes change as you go down the group? And how does this affect the rate of reaction?

A
  • Bond enthalpy decreases as you go down the group because as you go down group 7.
  • As the C-X bond becomes weaker, the rate of reaction increases.
28
Q

True or False

When the N+ is stabilised in the second phase of nucleophillic substitution of halogenoalkane using NH3, the N+ GAINS two electrons.

A
  • False!!
  • The N+ gains 1 electron as one of the electrons in the covalent bond already belongs to N+, it gains the electron from the hydrogen.
29
Q

How do you name an amine when the NH2 is in the middle of the chain?

A
  • Name with numbered “amino” prefix
30
Q

Where will the nitrile functional group always be??

A
  • The C in nitrile functional group will always be C-1 (just like in carboxylic acids.)

() - extra info for clarification

AS Organic CHEM (6 decks)

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