R3.4 - electron-pair sharing reactions Flashcards
(44 cards)
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
