CH0003 - lecture 2 - General Reactivity in Organic Chemistry
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What are electrophiles?
An electrophile (literally electron-lover) is a reagent attracted to electrons
Electrophiles accept electron pairs, therefore they are Lewis acids(see acid-base reaction theories).
Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.
What is a nucleophile?
A nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction.
Nucleophiles donate electrons, therefore they are by definition Lewis bases.
All molecules or ions with a lone pair of electrons (or covalent bonds!) can act as nucleophiles.
How do electrophiles and nucleophiles relate to each other?
Like REDOX, for a chemical reaction to occur with a electrophile or nucleophile the other must be present too.
Nucleophiles react with electrophiles!
What are curly arrows?
Curly arrows (and that's exactly what they are called!) are used in mechanisms to show the various electron pairs moving around.
They are used to explain reactivity in a simple way.
- The arrow tail is where the electron pair starts from, an electron pair either: a bond or a lone pair.
- Remember that a lone pairis a pair of electrons at the bonding level which isn't currently being used to join on to anything else.
The arrow head is where you want the electron pair to end up. i.e. to the electrophile, NOT vice versa
What does the arrow head tell us on curly arrows?
As we are talking about pairs of electrons moving/reacting – we use a double headed arrow. One for each electron, this avoids confusion with other types of reactions – (Radicals…… see later)
What can bonds be?
Bonds can be nucleophilic, after all they are a source of electrons. Particularly if they are a pi bond (more electron rich than a sigma bond).
What can electrophiles be?
Electrophiles can be charged species, such as protons (H+), carbocations (H3C+) or polar atoms (positive)
i.e. the carbon in H3C-Cl because chlorine in very electronegative (2.5 vs 3.5).
What are free radicals?
Free Radicals – the species only have one electron for the reactivity, albeit a very reactive one. Use a single headed arrow!
What are elimination reactions?
An elimination reaction is where two substituents are removed from within a molecule in either a one or two-step mechanism (E1 and E2)
How do we know whether or not a reaction is SN1 or 2
Initially someone had to do all of the experiments and measure rate data i.e how fast the reactions proceeded. Then vary concentrations of reactants.
The experimental data provided the “rate expression” what effects the reagents have on the rate
What is an SN2 reaction?
The number 2 means both reactants are in the rate equation i.e. changing the concentration of either reagents affects the rate.
- The SN2 reaction is a type of reaction mechanism that is common in organic chemistry.
- In this mechanism, one bond is broken and one bond is formed synchronously, i.e., in one step. SN2 is a kind of nucleophilic substitution reaction mechanism.
- Since two reacting species are involved in the slow (rate determining) step, this leads to the term substitutionnucleophilic (bi-molecular) or SN2, the other major kind isSN1
What is a nucleophilic substitution?
In organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a so-called leaving group; the positive or partially positive atom is referred to as an electrophile.
- The most general form for the reaction may be given as
- Nuc: + R-LG → R-Nuc + LG:
The electron pair (:) from the nucleophile(Nuc) attacks the substrate (R-LG) forming a new bond, while the leaving group (LG) departs with an electron pair. The principal product in this case is R-Nuc
What is an SN1 reaction?
- Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile).
- An sp3-hybridized electrophile must have a leaving group (X) in order for the reaction to take place.
What is the general formula of the SN1 mechanism?
What is the general formula for the SN2 mechanism?
What is nucleophilic addition?
In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electron deficient or electrophilic double or triple bond, a π bond, reacts with electron rich reactant, termed a nucleophile, with disappearance of the double bond and creation of two new single, or σ, bonds.
What is electrophilic addition?
- In organic chemistry, an electrophilic addition reaction is an addition reactionwhere, in a chemical compound, a π bondis broken and two new σ bonds are formed.
- The substrate of an electrophilic addition reaction must have a double bond or triple bond.
What is resonance?
In chemistry, resonance is a way of describing how delocalized electrons within certain molecules can behave
A molecule with such delocalised electrons can be described by several different contributing structures
These structures are derived by moving electron pairs around – ‘pushing curly arrows’
The real structure, resonance hybrid, of such systems is an average of the total number of resonance forms possible (if symmetrical then equal, if not then ‘weighted’)
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