Chapter 5: Reactions, electron flow and resonance Flashcards
reaction equations and reaction mechanisms
- a reaction equation describes which products are formed
- reaction mechanisms describes how the reaction happened by showing the movement of electrons
electrons flowing away
- negative to neutral
electrons flowing towards
- neutral to negative
over the arrow notation in chemical reactions
- extra details/ info below and above the the arrow
- there is no formal rules about what to show
- if desired lots of details can be added
- reagents/solvents (equivalents), concentrations, solvents temperature and time
drawing reaction mechanisms
- reaction mechanisms are described using curved arrows that represent the movement of electrons
- in all cases the arrow starts where the electrons are and ends where the electrons will be
identifying areas of importance
- when drawing a reaction mechanism it is useful to recognize areas with a high amount of electron density (this electron density may be able to create a new bond)
- curved arrows typically start at:
- heteroatoms (with lone pairs)
- anions (with lone pairs)
- pi (bonds)
- curved arrows typically end at” S+ atom from polar bonds+
- an atom bonded to a cation Pi (bond)
general tips for curved arrows
- curved arrows show the movement of electrons
- if a new bond is forming the arrow will point to the atom it is being formed to
- ## if a bond is becoming a lone pair the arrow will point to the atom that gets the lone pair
making or breaking a pi bonds have two conventions
- under the old convention the arrow points to the area where the new bond will be
- under the new convention the arrow points to the atom that the new bond is to (most people dont use new convention)
when electrons move the formal charge on atoms might change
- when an arrow ends at an atom that atom is gaining electron density (its formal charge may decrease)
- when an arrow points away from an atom that atom is losing electron density (its formal charge may increase)
- if an arrow points to an atom and another arrow points away, usually their effects cancel
when drawing mechanisms its often helpful to
- explicitly draw lone pairs
- conserve balanced equations throughout (atoms and charges)
reaction coordinates
- energy diagrams = relative energues of species in a reaction
starting materials
compounds before the reactions
intermediates
are compounds made during the reaction that continue undergoing transformations to eventually form the products
products
the new compounds made at the end of the reaction
transition states
- as starting materials become intermediates and as intermediates become products, they pass through transition states
- they are not a different kind of compound, they are what compounds look like when undergoing a transformation
- for each step in the mechanism there will be a transition state
activation energy
- the energy difference between a starting material/intermediate and the transition state beside it = delta G
- the larged delta G corresponds to the rate-determining step = rate limiting step (making it not the step sometimes with the higher transition state energy
exergonic
- if the products are lower in energy than the starting materials
endergonic
- if the products are higher in energy than the starting materials
practical use of reaction coordinates
- determine how many steps are in a reaction/mechanism (step per transition rate)
- determine which step is rate limiting
- determine which species are favoured at equilibrium (acid - base reactions)
resonance structures
- some molecules can be represented as different lewis or line-angle structures that have the same placement of atoms but a different placement of electrons
- electrons are not moving around one bond/lone pair to another
- attmepts to approximate the real distribution of electron density in the molecule
- curved arrows are often used to make it easier to understand the relationship between resoncance strutures
a drawing trick to understand resonance better
- they represent imaginary electron movement
- resonance is not a physical process, electrons are not moving along the paths of the curved arrows
- molecules do not look like one form one second and another the next
- molecules look like a weighted superposition of all resonance forms at once
stabalization by resonance/delocalisation
- having multiple resonance forms provides a large amount pf stabilization (lowers the energy required to exist)
- usually the more resonance forms a group or molecule has the more stability it gets
requirements for having resonance forms
- because having resonance forms is very stabilizing if a molecule can have resonance fors it (almost) always will
- there are several features/patterns that make resonance possible
- it is usually easier to memorize these at first and then practice with drawing resonance forms
resonance is possible if there is a polar bond
- often a carbon double or triple bonded to a heteroatom
- electrons will move to more electronegative side
requirements for having resonance forms
- it is possible if there is a pi bond directly connected to an atom with a lone pair (never ever breaking single bonds)
- a pi bond directly connected to an atom with an incomplete octet
- two double bond separated by another bond
general tips for drawing resonance structures
- always draw in lone pairs
- do not move atoms
- do not move sigma bonds
- do not break octet rule
evaluating contributions of resonance forms
decreasing in order of importance
1. the most atoms with full octets
2. fewest formal charges
3. negative formal charges on more electronegative atoms
4. positive formal charges on less electronegative atoms
5. similar charges are as far apart as possible
6. opposite charges are as close as possible
determine hybridization with multiple resonance forms
- depending on which resonance form is being looked at the hybridization of atoms might seem to be different
- resonance is not a physical process, atoms do not move
