R3.4 - electron-pair sharing reactions Flashcards
What is a nucleophile?
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.
What are examples of neutral nucleophiles?
H2O, NH3, ROH, RNH2
What are examples of anionic nucleophiles?
OH-, F-, Cl-, Br-, I-, CN-, R-
What happens when nucleophiles donate electrons?
When nucleophiles donate electrons to other species, it forms a coordination bond, which is a covalent bond.
What occurs in nucleophilic substitution reactions?
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.
What are some examples of nucleophilic substitution?
CH3Cl + OH- –> CH3OH + Cl-
C3H7Br + OH –> C3H7OH + Br-
What are the properties of halogenoalkanes?
- 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.
Why are halogenoalkanes useful?
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.
What is the overall reaction between a nucleophile and halogenoalkane?
Nucleophile + Substrate –> Electrophile + Leaving group
What is a leaving group?
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).
Why are halogens good leaving groups?
Halogens are good leaving groups as they form relatively weak bonds with C.
Why does a higher electronegativity make a halogen more susceptible to a nucleophilic attack?
A higher electronegativity means bonded electrons are drawn to the halogen atom, making the C electron-deficient.
What happens if the nucleophile reacts in a neutral species and what is the overall reaction?
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
What is heterolytic fission?
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.
Which atoms gain a negative charge?
The most electronegative atom.
What are some examples of heterolytic fission?
Cl-Cl –> Cl+ + Cl-
H-Cl –> H+ and Cl-
What is the opposite process to heterolytic fission?
The opposite process occurs when a nucleophile donates a pair of electrons to an electrophile, forming a coordination bond.
What is an electrophile?
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.
What are examples of neutral electrophiles?
HX, X2, H2O, RX
What are examples of cationic electrophiles?
H+, NO2+, NO+, CH3+, R+
What are alkanes?
Unsaturated hydrocarbons that undergo addition reactions
Why are alkanes more reactive than alkenes?
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.
Why are alkanes prone to electrophilic attack?
High electron density of the double carbon bond leads to electrophilic addition reactions.
What is the addition reaction of alkanes with water (hydration)?
Hydration - converts alkanes into alcohol using water (poor electrophile), so it requires a strong acid catalyst, such as sulfuric acid.
CH2CH2 –> CH3CH2OH
Why is hydration of ethene important?
Hydration reaction of ethene is important as ethanol is a very important solvent and is manufactured on a large scale.
What is the addition reaction of alkanes with halogens?
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.
How do alkanes and alkanes react with bromine water?
Alkanes react with bromine water through a radical mechanism, requiring UV.
Alkenes do not need UV as the pi bonds react readily with bromine.
What is the addition reaction of an alkane with hydrogen halides?
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.
What are the 3 possible addition reactions that an alkane can undergo?
- Alcohol
- Dihalogenoalkane
- Halogenoalkane
What is a Lewis acid?
Electron pair acceptor (electrophile)
What is a Lewis base?
Electron-pair donor (nucleophile)
What happens when a Lewis acid and a Lewis base react?
Form a coordination bond
What is an example of a Lewis acid and Lewis base reaction?
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.
When are coordination bonds formed?
Coordination bonds are formed when ligands donate a pair of electrons to transition metal cations, forming complex ions and a covalent bond.
What do ligands and transition metals act as when coordination ions are formed?
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.
What are examples of neutral ligands?
H2O, NH3, CO
What are examples of anionic ligands?
I-, Br-, F-, Cl-, OH-, SCN-, CN-
What must ligands contain?
At least 1 lone pair of electrons
What are the shapes and numbers of the 4 different complex ions?
Linear - 2
Square planar - 4 (different ligands)
Tetrahedral - 4 (same ligands)
Octahedral - 6
What does charge of a complex ion depend on?
- Charge on central metal ion
- Charge on ligand
- Coordination number
How do you work out charge on a central metal ion?
Charge on central metal ion = Charge on complex - Charge on ligand
What are polydente ligands?
Ligands that contain more than 1 lone pair, and so can form multiple coordination bonds with the central metal ion.
What is a chelating agent?
EDTA4- - when 1 ligand can form all 6 coordination bonds with the central metal ion, occupying all 6 potential octahedral sites.
What are the uses of chelating?
Used to remove poisonous transitional metals from solutions and inhibits enzyme-catalysed oxidation reactions.
