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A curly arrow moves a pair of electrons from its tail, on a lone pair or bond, to its head, where a bond or lone pair forms; charges follow the arrows, and an arrow never starts on an atom or a positive charge.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will say what an arrow shows, read arrowheads, spot wrongly drawn arrows, recount charges after an arrow, and choose the arrows for a one-step substitution.
You can name the nucleophile and the electrophile in a reaction. A curly arrow draws the step between them: which electrons move and where they go.
A curly arrow shows a pair of electrons moving, from its tail to its head. A fishhook (half-headed) arrow moves one electron. A mechanism is a reaction written as a sequence of arrows.
Every arrow obeys three rules:
In CH3Br + CN− → CH3CN + Br−, one arrow runs from cyanide's lone pair to the carbon (a new C–C bond) and one from the C–Br bond to bromine (the bond breaks, bromine leaves as Br−). Cyanide's −1 is used up; bromine gains it.
Another way: steps
To check an arrow:
Formal charge bookkeeping checks a mechanism. When water's oxygen gives a lone pair to a carbocation, it goes from two bonds and two lone pairs to three bonds and one lone pair: $6 - 2 - 3 = +1$. The carbon's positive charge has moved onto oxygen, exactly as the arrow predicts. If a mechanism's charges do not add up to the same total before and after each step, an arrow is wrong. Radical reactions use fishhook arrows, each moving one electron, and are a different family of mechanisms.
A curly arrow shows an atom physically moving. It shows electrons.
An arrow can start on H+. A proton has no electrons; arrows point to it.
One full arrow moves one electron. It moves two.
Direction doesn't matter. Electrons flow from tail to head.
Arrow 1: from a lone pair on hydroxide's oxygen to the acidic hydrogen.
Tail on electrons.
Arrow 2: from the O–H bond of the acid to its oxygen.
The old bond breaks.
Hydroxide becomes water, and the acid becomes ethanoate with the −1 charge.
Charges follow the arrows.
Where does the first arrow start?
On ammonia's lone pair.
Where does the second arrow start and end?
From an O–H bond of H3O+ onto its oxygen, giving water.
In a mechanism, a curly arrow runs from a lone pair on the oxygen of hydroxide to the carbon of bromomethane. What does it show?
Match each feature of an arrow to what it means.
| a pair of electrons moves | one electron moves | a new bond forms there | that atom gains a lone pair | |
|---|---|---|---|---|
| a full, two-barbed arrowhead | ||||
| a half-headed fishhook arrow | ||||
| a head ending between two atoms | ||||
| a head ending on a single atom |
A student's mechanism contains four arrows, described below. Mark every arrow that is drawn correctly.
This task has no paper form; do it on a device.
An arrow runs from one lone pair on the oxygen of water to the empty orbital of a carbocation, making a new O–C bond. What is the formal charge on that oxygen afterwards?
Answer:
In the one-step reaction CH3Br + CN− → CH3CN + Br−, which pair of arrows describes it?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
In the addition of HBr to ethene, three arrows are drawn: (1) from the C=C to the H of HBr, (2) from the H–Br bond to Br, (3) from a lone pair on Br− to the carbocation's carbon. Say what kind of electrons each arrow starts on.
| starts on | |
|---|---|
| arrow 1 | |
| arrow 2 | |
| arrow 3 |
You can read a mechanism's arrows. Tell someone why an arrow never starts on H+. Next: what happens between the arrows — intermediates and transition states.
8. Your turn: ammonia attacking a proton of H3O+, step 3