Opposites Attract Relationship
within Chemistry: Curved
Arrows, Electrophiles, and
Nucleophiles
Define the following terms:
Reaction Mechanism:
Nucleophiles :
Electrophiles:
Curved Arrows:
Brosted Lowry Base:
Brosted Lowry Acid:
Carbocation:
Answers:
Reaction Mechanism: a step by step account of all the electrons and bond changes in a chemical
reaction.
Nucleophile: a species that shares a pair of electrons with another atom to form a new covalent
bond; Lewis base
Electrophile: A species that gains a pair of electrons from another atom to form a new covalent
bond; a lewis acid
Curved Arrows: indicate flow of electrons
Brosted Base: Proton acceptor (i.e HO- aka: hydroxide ion)
Brosted acid: Proton donor (i.e H2SO4)
Carbocation: structure with positive formal charge on carbon due to open octet.
Basic Concepts:
Mechanisms:
In Chemistry, one of the most important topics is mechanisms. Mechanisms are when two
compounds react with each other to form new and different products. Within Mechanisms, the
movement of electrons is shown which is quite important since every reaction involves the flow
of electron density (electrons are needed to break or form new bonds). To map out the flow of
electrons curved arrows are used.
Curved Arrows:
These curved arrows indicate how the reaction took place by representing the flow of an electron
pair.
Remember that even though these arrows look a lot like the arrows used in resonance, they are
not the same!!!
In resonance, the resonance structures do not exist because they are continuously altering
from one to another.
In mechanisms, however, the curved arrows actually show the movement of electrons.
Each arrow has a tail and a head. The tail is the location of the electrons and the head is where
they are going. In other words, it shows the electrons coming from the electron rich area to the
electron poor area.
TAIL
Electron Source:
where electrons are
coming from
HEAD
Electron
Destination:
where
electrons are
being moved
3 possible ways electrons can move:
1. Lone pair  bond
2. Bond lone pair
3. Bond bond
1. Lone pair bond
The tail of the arrow shows that the electrons are coming from the
oxygen atom and are going to form a bond with the carbon atom, as
shown by the head of the arrow.
•
Also take note of the changes in formal charge on the oxygen and
carbon atom
*Could be seen as this
where the purple dotted
line shows the bond that is
about to form*
wher
2. Bond Lone Pair
The tail of the arrow is on the bond and the head of the arrow is pointing towards the
bromine atom. This indicates that the bond is breaking and those electrons from the
bond are going solely towards the bromine atom. This causes the carbon atom to loses
one electron, making its formal charge +1 and the bromine atom has gained one
electron, making it -1.
Carbon with positive
charge = Carbocation
Called A bromide
atom (-ide shows (-)
charge)
3. Bond  bond
In this process there are usually two arrows which show the flow of electrons. First
the blue arrow shows the electrons from the pi bond forming a bond with the
hydrogen atom. Then since the hydrogen atom cannot have two bonds, the green
arrow shows the electrons going from the bond between hydrogen and chlorine
solely to the chlorine atom.
There can also be mechanisms that have two of the above steps:
NEVER show the arrows going
in opposite directions because
this indicates that the electrons
are flowing in opposite
directions at the same time.
Practice Problems
In each of the following molecules, classify the arrows shown as one of the three types
described above. (bondbond OR bondlone pair OR lone pair bond)
11111111,njfklfkefakjhj
1.
4.
Answers:
4.
Nucleophilic vs. Electrophilic
Many reactions are activated by attraction between opposite charges (Electrostatic).
Electrostatics which has to do with this attraction can be described by talking about
nucleophiles and electrophiles or in other words something that is electron rich
interacting with something that is electron deficient.
Nucleophilic: literally translated as “Nucleus loving”, a species donates its
electrons to electrophiles and forms a covalent bond. It has electron density that is
available to share, which is usually in the form of a pi bond or lone pairs or wants to
share because of formal charge.
•
•
•
•
Electron rich
On the Blunt end of arrow
Lewis base: electron pair donator
o Stronger base=stronger nucleophile
Will usually have pi bonds or lone pairs so that it can form a bond with
electrophiles
Electrophiles: literally translates to “electron loving”, is a species
gains electrons from nucleophiles to form a covalent bond. It is electron
deficient and wants to gain electrons.
Further details:
• Electron poor which is usually shown by: + charge, open octet
• On the pointy side of arrow
• Lewis acids: electron pair acceptors
o Bronstead acid is also an acceptor
Practice Problems:
Part 1: Classify each compound as Nucleophile or Electrophile.
1.
2.
..
..
.
.
3.
4.
..
.
5.
Part 2:
Determine which Compound is the Nucleophile and which is
the Electrophile.
2.
1.
4.
3.
Part 3: Complete the Mechanisms by drawing in the appropriate
arrows.
Answers:
Part 1:
1.Nucleophile
2.Nucleophile
3.Electrophile
4.Nucleophile
5. Electrophile
Part 2:
1.
3.
Part 3:
2.
4.
Hope this helped!! Good Luck studying!
Works Cited
Klein, David R. Organic Chemistry as a Second Language. Danvers: John Wiley, 2004.
Print.
Hardinger, Steven. Chemistry 14C Lecture Supplement. 9th ed. Plymouth: Hayden-McNeil,
2011. Print
Hardinger, Steven. Chemistry 14C Thinkbook. 6th ed. Plymouth: Hayden-McNeil, 2008.
Print.
Google images for many images included in the tutorial