Nucleophillic Substitution Flashcards
What do mechanisms show?
- Mechanisms show the movement of pairs of electrons.
How is the formation of a covalent bond shown by mechanisms?
- Formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond.
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How is the breaking of a covalent bond shown by mechanisms?
- Shown by a curly arrow starting from the middle of a bond (to another atom.)
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Where do curly arrows always start?
- Curly arrows always start from an area with electrons ie. a bond/ lone pair of electrons.
What is a nucleophile?
- A nucleophile is a species which donates a pair of electrons to an electron-deficient centre, forming a dative covalent bond.
Give 3 examples of nucleophilles.
- Ammonia (:NH₃)
- Cyanide ions (:CN⁻)
- Hydroxide ions (OH⁻) –> have two lone pairs of electrons.
What type of bonds do halogenoalkanes have? Explain why this is the case
- Polar bonds.
- Because halogens are more electronegative than carbon , so the halogens will draw the electrons in the covalent bond closer to themselves.
What allows halogenoalkanes to be able to be attacked by nucleophilles?
- The POLAR BOND means that halogenoalkanes can be attacked by nucleophiles.
What is the reaction called when nucleophilles (OH-/ CN-/ NH3) attack halogenoalkanes ?
- Nucleophillic substitution
What are the conditions for the reaction of nucleophillic substitution between halogenoalkanes and hydroxide ions?
- Warm aqueous NaOH (source of OH- ions.) (warm will be achieved by heating reaction mixture.)
- Carried out under reflux
What does “carried out under reflux” mean?
- Condenser on top of test-tube so if any substance evaporated (due to the heat), it will be condensed straight back into reaction mixture.
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.
- 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.
Overall equation for nucleophillic substitution using NaOH. NAME PRODUCTS
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
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What is the role of OH-/ CN- in nucleophillic substitution reactions?
- They are nucleophilles.
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.
- 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.
What are the conditions for nucleophillic substitution of halogenoalkane using CN- ions?
- Warm ethanolic potassium cyanide (source of CN- ions.)
- Carried out under reflux.
Overall equation for nucleophillic substitution of halogenoalkane using CN- ions. NAME THE PRODUCTS
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
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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.
- 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.
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.
- 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.)
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Why do we call the NH3 molecule (in the second stage of nucleophillic substitution using NH3) a base?
- Because bases will accept protons so will react with H+ ions.
Conditions required for nuelcophillic substitution of halogenoalkane using NH3?
- Heat with ethanolic ammonia.
- Must have an excess of ammonia.
Why must we have excess of ammonia in nucleophillic substitution reaction of halogenoalkane using NH3?
- So that there’s enough ammonia for 2nd molecule to act as a base and react with hydrogen.
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?) What are the products ?
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
What does NH3 act as in nucleophillic substitution of halogenoalkane?
- First molecule in first phase acts as nucleophille.
- Second molecule in second phase acts as base.
What type of reaction is nucleophillic substitution of halogenoalkane using NH3?
- Reversible reaction
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?
- 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
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?
- 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.
True or False
When the N+ is stabilised in the second phase of nucleophillic substitution of halogenoalkane using NH3, the N+ GAINS two electrons.
- 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.
How do you name an amine when the NH2 is in the middle of the chain?
- Name with numbered “amino” prefix
Where will the nitrile functional group always be??
- The C in nitrile functional group will always be C-1 (just like in carboxylic acids.)
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