Class Activity 2 Lv'""' ~ ~t i:n~ t doc"~~' Resonance Structures Two or more Lewis structures differ only in the placement of electrons.

(1)

CHEM301, Fall 2010 Dr. Ruder 1

Class Activity 2

Lv'""'

~ ~t i:n~ t

doc"~~'

Resonance Structures

Two or more Lewis structures differ only in the placement of electrons.

Model 1: Resonance Structures for a Carbonyl Group

..

Questions:

1. (a). How many total electrons are in structures I ( ) and II )? Is the total number of electrons in each structure the same? (yes or no)

)?

(b). How many single bonds are in structures I ( ) and II ('---~

Is the number of single bonds the same in each structure? (yes or no)

)? (c). How many lone pair electrons are in structures I ( ) and II('---~

Is the number of lone pair electrons the same in each structure? (yes or no)

)?

(d).

How many pi electrons are in structures I ( ) and II('---' Is the number of pi electrons the same in each structure? (yes or no)

(e). Based on the answers above, describe what is different between structures I and II (in terms of electrons)?

2. Is the net charge of structure I the same as the net charge of structure II? (yes or no)

3. What do the curved arrows show in the above structure?

Model 2:

Rules for Drawing Resonance Structures:

1. Move lone pair electrons on an atom to form a new double bond to that atom (arrow a).

2. Move double bond electrons to make a lone pair on one of the atoms of the original double bond (arrow b).

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CHEM301, Fall 2010 Dr. Ruder

4. For the following resonance structures of methacrylic acid below:

C)=

:f:e

HC/C""-.OH

~

.. - - •

~&""-o'H

II

\._~

. ..

..

C~Ill C~ W

·o·e

_{.I .}

_,.___ HC/c~OH

II

®

CH2 VI

(a). Label each arrow as either arrow a, b, or c, according the rules listed in Model2.

(b). Is the total charge for each resonance form the same? (yes or no)

(c). In converting structure V to VI, it is necessary to move two pairs of electrons. Draw the resonance form if only the lone pair electrons on oxygen were moved, as indicated by the arrow below. Explain why this is not a valid resonance form.

2

5. (a). In structure Ill, the double bond electrons move to form a lone pair on oxygen as shown in Model 2. The arrow below shows the double bond electrons moving to form a lone pair on carbon. Draw the resulting resonance form (make sure to put in charges).

·o.

"11-7

_..

• _...c, ••

HC'. "'OH

II

CH2 III

(b). Although this form is possible, it is not likely. As a group, discuss why the lone pair is more likely to form on oxygen than on carbon.

6. The following items must always be followed when drawing resonance forms: "'Total charge must always be the same

"'Total number of electrons must stay the same

"'Atoms must not have more than eight electrons (octet rule) "' Single bonds must not be broken

"' Atoms should not be moved

As a group discuss whether each resonance pair below shows a correct resonance structure. If an incorrect structure is shown, indicate which of the above rules has been violated.

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- - ----~

~·---CHEM301, Fall 2010 Dr. Ruder

Model

3: Stability of Resonance Forms

Not all resonance forms have the same energy and some forms may be more stable than others. The major resonance contributor is determined by these factors (the most important first):

1. complete octet

2. as many bonds as possible

3. negative charge resides on more electronegative atom 4. small charge separation

:0:

_{:0: .}

.. e

. 0.

.. e

II

_I

. .

_I

...-c, ..

..

_{/c, ..}

..

_/C~<±) R OH

..

_R _<±) _OH _R _OH VII _VIII

..

IX

.

Questions:

7. Add curved arrows to show the conversion of VII ~ VIII and VIII ~ IX in Model 3.

3

8. Check all atoms in Model3 for a complete octet: circle atoms that do not have complete octets. 9. Do all resonance forms in Model 3 have the same number of electrons? (yes or no)

10. Do all resonance forms in Model3 have the same total charge? (yes or no)

11. Based on the rules in Model 3, which structure would you expect to be the major contributor? Explain.

12. For the following structure, use curved arrows to draw three additional resonance structures and predict which form, if any, would be the major contributor. (HINT: move double bond or lone pair electrons only and try moving one pair at a time)

Reflection: on a separate sheet of paper.

As a group, describe three concepts your group has learned from this activity and the one most important unanswered question about this activity that remains with your group. Place this in your group folder before leaving class.

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--~~---CHEM301, Fall 2010 Dr. Ruder ₁

Class Activity 2

Resonance Structures

Two or more Lewis structures differ only in the placement of electrons.

Model 1: Resonance Structures for a Carbonyl Group

Questions:

1. (a). How many total electrons are in structures I ( l~ ) and II ( 7..~ )? Is the total number of electrons in each structure the same? t!§j>cr no)

(b). How many single bonds are in structures I ( ~ ) and II (

f/

)?

Is the number of single bonds the same in each structure? (yes or~

(c). How many lone pair electrons are in structures I (

'f

)

and II (

3

)? Is the number of lone pair electrons the same in each structure? (yes o~

(d). How many pi electrons are in structures I ( 1.- ) and II (

0

)?

Is the number of pi electrons the same in each structure? (yes or<!!9:)

(e). Based on the answers above, describe what is different between structures I and II (in terms of electrons)? .sqW\4.

o...-.uw.t

~

e-

f, ... t

ot,.(f~r

4mo ...

t1fs

01,-e 1/,/'VV-

Pe.t•<J

01..,J

1/oi~of...(__blNit:l!

2. Is the net charge of structure I the same as the net charge of structure

II'~or

no) 3. What do the curved arrows show in the above structure? ~

4./

JOJ ._,_

IY'l/l..At~Vta,il I ICi.Cl~t

i

e

~

Model 2:

Rules for Drawing Resonance Structures:

1. Move lone pair electrons on an atom to form a new double bond to that atom (arrow a).

2. Move double bond electrons to make a lone pair on one of the atoms of the original double bond (arrow b).

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4. For the following resonance structures of methacrylic acid below:

b

(_)=

:f:8

HC/C'-...OH ••-••

~&"'-o'H

II

c."-~

..

C~Ill C~ N

(a). Label each arrow as either arrow a, b, or c, according the rules listed in Model 2. (b). Is the total charge for each resonance form the same?/t'Ye5)or no)

(c). In converting structure V to VI, it is necessary to move'iWc>pairs of electrons. Draw the resonance form if only the lone pair electrons on oxygen were moved; as indicated by the arrow below. Explain why this is not a valid resonance form .

.. 8

·o· -

0 .l.n

~~=<=>

...-:C..._. •• ~ C """ HC-7" "OH ..::; ~ -..:.J

I

we

_I~

~ -~~

®

CH2 V C1 c.112.

$

foMCIJ

_I 2

5. (a). In structure Ill, the double bond electrons move to form a lone pair on oxygen as shown in Model 2. The arrow below shows the double bond electrons moving to form a lone pair on carbon. Draw the resulting resonance form (make sure to put in charges).

·o .

(£')

'11-7

·'f.

f>

/c .. .. ..

11c.

/c.'-~

HC"' "-oH I _I _..O _ij

II

C~+v

CH2 III

(b). Although this form is possible, it is not likely. As a group, discuss why the lone pair is more likely to form on oxygen than on carbon. ov.. ~ 1c ...,., ... -oc.t

t

....

b...o

c'·

L. 1.

,-, r . ~ • ... :.t: C...•rro 0" VI~~ fU&

rue-.

It~ e~J

i

6. The following items must always be followed when drawing resonance forms:

...J Total charge must always be the same

...J Total number of electrons must stay the same

...J Atoms must not have more than eight electrons (octet rule)

...J Single bonds must not be broken

...J Atoms should not be moved

As a group discuss whether each resonance pair below shows a correct resonance structure. If an incorrect structure is shown, indicate which of the above rules has been violated.

')(

&

[

=TI~

c

- -

=me> ]

c

.

/ '-....H H / '-....H

f

H

f1,J7<H

I

[ >=< __.

H H H

J=c\

H

l

(6)

Model 3:

Stability of Resonance Forms

Not all resonance forms have the same energy and some forms may be more stable than others.

The major resonance contributor is determined by these factors (the most important first): 1. complete octet

2. as many bonds as possible

3. negative charge resides on more electronegative atom

4. small charge separation

:0:

·o·

_e

. 0.

.. e

t..ll

.

_IC\

.

. .

_I

/ c ,

..

4

..

R/~'-....OH

4

..

/C~®

R OH _R _OH VII _VIII

..

IX

. .

Questions:

7.

Add curved arrows to show the conversion of VII~ VIII and VIII~ IX in Model3.

3

8.

Check all atoms in Model 3 for a complete octet: circle atoms that do not -~Q)(e complete octets.

9.

Do all resonance forms in Model 3 have the same number of electrons? ~or no)

10.

Do all resonance forms in Model3 have the same total charge? ~or no)

11. Based on the rules in Model 3, which structure would you expect to be the major contributor? Explain.

~-12. For the following structure, use curved arrows to draw three additional resonance structures and predict which form, if any, would be the major contributor. (HINT: move double bond or lone pair electrons only and try moving one pair at a time)

14 1-J tr- I ,, ~

'

c

<::>' \

c..-::;:;

~-· / c.-c~. ~

I

~: 11

Reflection: on a separate sheet of paper.

As a group, describe three concepts your group has learned from this activity and the one most important unanswered question about this activity that remains with your group. Place this in your group folder before leaving class.

I