Note: Look for the boxes like the one below. If clicked on the gray text, it will expand to show the content of the explanation or exercise. To collapse it back, click on the gray text again.
⊕ Interactive explanation/exercise (click to show)
That would be the content of the explanation or exercise. Animated explanations go step by step. Click "play" button (▶)to advance a step, or replay button (↵) to replay the last step
Reactions
Reading reactions is equally important in poker and in organic chemistry.* Compounds "react" to being treated with other compounds or being subjected to particular conditions: by changing into other compounds. A reaction may look like this:
compound1 + compound2 = compound3 + compound4
That means: compound1 reacts with compound2. They change into compounds 3 and 4.
Compounds 1 and 2 would be called "reactants" or "starting materials", while comppounds 3 and 4 - "products".
It can be viewed as "Compound1 is treated with compound2 to start the reaction". Compound2 can then also be called "Reagent", while compound1 - "substrate" .
Often times, another compound, e.g. compound5, would be involved - it does not change into anything, but without it, compounds 1 and 2 do not react with each other. Compound5 would then be called "catalyst".
In organic chemistry reactions are usually written slightly differently. For instance, below is an example of Hydration of alkenes and Oxymercuration/demercuration (hover over parts to see the description):
That means: reactant (on left) react with H2O and H2SO4 at 20 °C to produce the product (on the right). (Note: H2SO4 happends to be a catalyst, and H2O a reagent in this case - but one can only know that from being familiar with this particular reaction)
This describes a sequence of two reactions (two steps). First, reactant (on the left) reacts with H2O and Hg(OAc)2. A product forms (not shown), which becomes the reactant for the second reaction when it is treated with NaBH4 to give the shown product on the right.
⊕ Possible variations (click to show)
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⊕ On two and more step reactions (click to show)
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Key transformation and other features of reactions
By comparing the product against the reactant, you can track down what changed in the reaction: which bonds were broken, which were made; which fragment is needed, and which is formed in the reaction. For instance, for the reactions above:
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The key transformation tends to be characteristic for a reaction: you can recognize the reaction by its transformation, and predict its product
(though some reactions may result in the same transformation, like the two above). Another key feature is the reagent. Many reactions use unique reagents
(e.g. Hg(OAc)2 squarely points to oxymercuration). Some more common reagents (such as H2SO4), however, are used in great
many reactions - in such case the reaction can only identified from a combination of reactants and reagents.
Once you definitively identify the reaction from a few pieces of information, you can expect all characteristic features and behaviours of this reaction. So then,
the name of the game is to learn features of all necessary reactions, learn to identify them by a few bits and pieces, and recognize how they play over in different cases.
⊕ An exercise#1 on transformations (click to show)
When a reaction is done with more elaborate reactants, multiple products may be possible, for instance, for hydration:
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Some reactions will form all possible products in nearly equal amounts, but others may have a preference toward some. If this preference is there, the reaction is
selective. In case of this particular transformation, one common organic reaction (oxymercuration-reduction)
will give preferentially one product, while another (hydroboration-oxidation) will
preferentially give the other. This type of selectivity, when possible products are regioisomers, and one
is preferred, is called regioselectivity.
⊕ Even more possible products: stereoisomers (click to show)
Possible products can also be stereoisomers (then, the selectivity between them would be stereoselectivity). For instance, in this reaction, for the second product:
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⊕ Terms for some common selectivities (click to show)
Regioselectivity of addition: Markovnikov vs. Non-Markovnikov (also called anti-Markovnikov)
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Regioselectivity of elimination: Zaitsev vs. Non-Zaitsev (also called anti-Zaitsev)
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Stereoselectivity of addition: syn vs. anti
Straightforward: syn prefers the product where the new groups (H and OH in this case) introduced to point in the same direction (either both up or both down),
anti - in opposite directions (one up, one down).
So, to make sense of most reactions you'd need to key learn features of a bunch of them (their transformation, reagents used, selectivity), and practice
to work with them. Some typical setups:
Reactant and reagents given, predict the product: identify the reaction from reactant and reagents, play out its transformation with its selectivity to get the product(s)
Reactant and product given, pick the reagent(s): identify transformation and its selectivity, pick the reaction they fit, use the appropriate reagent for this reaction
Reagents and product given, guess what the reactant was: identify the reaction from product and reagents. Play the transformation backwards, check if the selectivity fits
Below is a reference of reactions commonly studied in organic chemistry courses.
Me = CH3, Et = CH2CH3, Ac = COCH3, t-Bu = C(CH3)3, Ms = CH3SO2. Examples (hover over them to see the expansion):
Common conventions:
H2SO4 or others
strong acids (pka~2 or less) are catalysts in great many reactions. Besides H2SO4, many others can make an appearance, such as: TsOH, H3PO4, HClO4, etc. HCl, HBr, and HI can appear as well, but sometimes they are avoided as they do too much.
acid
largely the same to the one above, but usually means that the acid doesn't have to be as stong (something with pKa ~ 5 or below is usually OK)
H+
synonym of "acid", since acids release H+
H3O+
describes solution of acid in water (acids in water produce H3O+: H+ + H2O = H3O+). Most typically HCl in H2O, but could be varierty of others
base
most commonly something like NaOH or KOH, but could be many other bases: NaOEt, KOt-Bu, NaNH2, NaH, LDA, depending on the reaction
OH-
NaOH or KOH
X
when used as part of the structure, specified a halogen: Cl or Br or I
R
when used part of the structure, specified a generic "alkyl group" - CH3, C2H5, t-Bu, etc.
nucleophile
often written as Nu: or Nu, any compound that contains an atom with a lone pair (nucleophilic center), which most typically
O, N, S, C-. See substitution reaction for details (of which there are many)
Some common nucleophiles: Cl-, Br-, I-, H2O, OH-, CH3OH, NH3, -CN, CH3S-
The above works well and lets you deal with most reactiosn, but some situtations call for a deeper look (such as exceptions, special cases, some reactions
with large variability in reactant/reagent structure and outcomes, etc.). That deeper look is study of the mechanism of reactions.