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chemistry > R3.4 - electron-pair sharing reactions > Flashcards

R3.4 - electron-pair sharing reactions Flashcards

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1
Q

What is a nucleophile?

A

A reactant that forms a bond to its reaction partner (electrophiles) by donating both electrons. They are electron-rich and tend to have a negative charge.

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2
Q

What are examples of neutral nucleophiles?

A

H2O, NH3, ROH, RNH2

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3
Q

What are examples of anionic nucleophiles?

A

OH-, F-, Cl-, Br-, I-, CN-, R-

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4
Q

What happens when nucleophiles donate electrons?

A

When nucleophiles donate electrons to other species, it forms a coordination bond, which is a covalent bond.

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5
Q

What occurs in nucleophilic substitution reactions?

A

A nucleophile donates an electron pair to form a new bond as another bond breaks, leaving a producing group as the nucleophiles are attached to the electron deficient atom.

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6
Q

What are some examples of nucleophilic substitution?

A

CH3Cl + OH- –> CH3OH + Cl-
C3H7Br + OH –> C3H7OH + Br-

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7
Q

What are the properties of halogenoalkanes?

A
  • Polar compounds as the halogen atom is more electronegative than carbon, exerting a strong pull on the shared electrons in the carbon-halogen bond. This means that the halogen gains a partial negative charge and the carbon gains a partial positive charge and is said to be electron-deficient.
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8
Q

Why are halogenoalkanes useful?

A

They are useful intermediates within organic synthesis pathways as the halogens can be substituted by a wide variety of nucleophiles, so that halogenoalkanes can be converted into many different classes of compounds.

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9
Q

What is the overall reaction between a nucleophile and halogenoalkane?

A

Nucleophile + Substrate –> Electrophile + Leaving group

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10
Q

What is a leaving group?

A

For the nucleophile to donate an electron pair to the substrate and form a new bond, another bond must break, leaving a species break away (leaving group).

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11
Q

Why are halogens good leaving groups?

A

Halogens are good leaving groups as they form relatively weak bonds with C.

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12
Q

Why does a higher electronegativity make a halogen more susceptible to a nucleophilic attack?

A

A higher electronegativity means bonded electrons are drawn to the halogen atom, making the C electron-deficient.

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13
Q

What happens if the nucleophile reacts in a neutral species and what is the overall reaction?

A

If the nucleophile is a neutral species, the product formed from the substrate will be positively charged.
- However, it deprotonates and loses a H+ to give a neutral product.
- If the leaving group was a halide ion, this combines with H+ to form a hydrogen halide.
Overall equation = H2O + RX –> ROH + HX

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14
Q

What is heterolytic fission?

A

Breakage of a covalent bond when both bonding electrons remain with one of the 2 fragments, creating oppositely charged atoms, with an unequal assignment of electrons.

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15
Q

Which atoms gain a negative charge?

A

The most electronegative atom.

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16
Q

What are some examples of heterolytic fission?

A

Cl-Cl –> Cl+ + Cl-
H-Cl –> H+ and Cl-

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17
Q

What is the opposite process to heterolytic fission?

A

The opposite process occurs when a nucleophile donates a pair of electrons to an electrophile, forming a coordination bond.

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18
Q

What is an electrophile?

A

Reactant that forms a bond with its reaction partner (nucleophiles) by accepting both bonding electrons, forming a covalent bond. They are electron-deficient, so tend to have a positive charge.

19
Q

What are examples of neutral electrophiles?

A

HX, X2, H2O, RX

20
Q

What are examples of cationic electrophiles?

A

H+, NO2+, NO+, CH3+, R+

21
Q

What are alkanes?

A

Unsaturated hydrocarbons that undergo addition reactions

22
Q

Why are alkanes more reactive than alkenes?

A

Alkanes are more reactive than alkenes as the double bonds have a high electron density, making it attractive for electrophiles. The reaction occurs at the site of the double bond, where the pi bond is selectively broken, creating 2 new bonding positions on C and enable alkanes to undergo addition reactions with electrophiles.

23
Q

Why are alkanes prone to electrophilic attack?

A

High electron density of the double carbon bond leads to electrophilic addition reactions.

24
Q

What is the addition reaction of alkanes with water (hydration)?

A

Hydration - converts alkanes into alcohol using water (poor electrophile), so it requires a strong acid catalyst, such as sulfuric acid.
CH2CH2 –> CH3CH2OH

25
Q

Why is hydration of ethene important?

A

Hydration reaction of ethene is important as ethanol is a very important solvent and is manufactured on a large scale.

26
Q

What is the addition reaction of alkanes with halogens?

A

Halogens react with alkanes to form dihalogeno compounds, which occur quickly at room temperature and result in the colour loss of the reacting halogen.
- A halogen atom attaches to each of the 2 C atoms of the double bond.

27
Q

How do alkanes and alkanes react with bromine water?

A

Alkanes react with bromine water through a radical mechanism, requiring UV.
Alkenes do not need UV as the pi bonds react readily with bromine.

28
Q

What is the addition reaction of an alkane with hydrogen halides?

A

Hydrogen halides react with alkanes to produce halogenoalkanes at STP.
- The reactivity order is HI>HBr>HCl due to decreasing strength of the hydrogen halide bond down the group.
- HI reacts readily as it is the weakest.

29
Q

What are the 3 possible addition reactions that an alkane can undergo?

A
  • Alcohol
  • Dihalogenoalkane
  • Halogenoalkane
30
Q

What is a Lewis acid?

A

Electron pair acceptor (electrophile)

31
Q

What is a Lewis base?

A

Electron-pair donor (nucleophile)

32
Q

What happens when a Lewis acid and a Lewis base react?

A

Form a coordination bond

33
Q

What is an example of a Lewis acid and Lewis base reaction?

A

BF3 + NH3:
BF3 - incomplete octet, so acts as a Lewis acid to accept 2 electrons.
NH3 - acts as a Lewis base, donating its lone pair of electrons.
- Coordination bonds have the same bond length, strength and chemical reactivity as covalent bonds.

34
Q

When are coordination bonds formed?

A

Coordination bonds are formed when ligands donate a pair of electrons to transition metal cations, forming complex ions and a covalent bond.

35
Q

What do ligands and transition metals act as when coordination ions are formed?

A

Ligands - act as Lewis bases and donate a pair of lone electrons.
TM - act as Lewis acids and accepts lone pair of electrons with vacant d-shell orbitals.

36
Q

What are examples of neutral ligands?

A

H2O, NH3, CO

37
Q

What are examples of anionic ligands?

A

I-, Br-, F-, Cl-, OH-, SCN-, CN-

38
Q

What must ligands contain?

A

At least 1 lone pair of electrons

39
Q

What are the shapes and numbers of the 4 different complex ions?

A

Linear - 2
Square planar - 4 (different ligands)
Tetrahedral - 4 (same ligands)
Octahedral - 6

40
Q

What does charge of a complex ion depend on?

A
  • Charge on central metal ion
  • Charge on ligand
  • Coordination number
41
Q

How do you work out charge on a central metal ion?

A

Charge on central metal ion = Charge on complex - Charge on ligand

42
Q

What are polydente ligands?

A

Ligands that contain more than 1 lone pair, and so can form multiple coordination bonds with the central metal ion.

43
Q

What is a chelating agent?

A

EDTA4- - when 1 ligand can form all 6 coordination bonds with the central metal ion, occupying all 6 potential octahedral sites.

44
Q

What are the uses of chelating?

A

Used to remove poisonous transitional metals from solutions and inhibits enzyme-catalysed oxidation reactions.

chemistry (22 decks)

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