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NOTES: • Write out the notes from my website.

Use different types of note-taking methods to

help you recall info (different color

pens/highlighters, bullets, etc)

• When I lecture we will add more info, so

leave spaces in your notes

• DRAW ALL PICTURES, FIGURES, AND

WRITE OUT ANY PRACTICE

PROBLEMS/QUESTIONS.

• WE WILL ANSWER THEM TOGETHER.

So…LEAVE SPACES SO WE CAN

ANSWER QUES.

Summary (end of notes) :

1-2 Sentences of what you learned

Reflect

Question:

Reflect on

the

material by

asking a

question

(its not

suppose to

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answered

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notes)

Right side: NOTES!

Date

Skillbuilder #4.1

Title and

Highlight

Write out Question only

• We will practice the skill in class after

lecture

Left side: Skillerbuilder problems

READ pg. 133 then

take notes

Hydrocarbons –composed of

hydrogen and carbon

Hydrocarbons are either

saturated (lack pi bonds) or unsaturated

READ pg. 133-146

then take notes

In the early 19th century, organic

compounds were often named on a

whim

Many of these compounds were given

“common names”

In 1892 a group of 34 Europeans

chemist met in Switzerland and

developed a system to naming organic

compounds

The group became known as the

International Union of Pure and Applied

Chemistry (IUPAC).

IUPAC nomenclature - system of

naming organic compounds

IUPAC names include:

1. Parent name (longest carbon chain)

2. Names of substituents

3. Location of substituents

Rule#1: Identify the parent chain -

the longest consecutive chain of

carbons

If there is more than one possible parent

chain, choose the one with the most

substituents (Branches) attached

If the parent chain is cyclic, add the

prefix “cyclo”

134 CHAPTER 4 Alkanes and Cycloalkanes

Names produced by IUPAC rules are called systematic names. T ere are many rules, and we

cannot possibly study all of them. T e upcoming sections are meant to serve as an introduction to

IUPAC nomenclature.

Selecting the Parent Chain

T e f rst step in naming an alkane is to identify the longest chain, called the par ent chain:

Choose longest chain

In this example, the parent chain has nine carbon atoms. When naming the parent chain of a com-

pound, the names in Table 4.1 are used. T ese names will be used very often in this course. Parent

chains of more than 10 carbon atoms will be less common, so it is essential to commit to memory at

least the f rst 10 parents on the list in Table 4.1.

meth

eth

3 prop

4 but

5 pent

hex

7 hept

oct

9 non

dec

undec

dodec

tridec

tetradec

pentadec

eicos

triacont

tetracont

pentacont

hect

TABLE 4.1 PARENT NAMES FOR ALKANES

NUMBER OF

CARBON ATOMS

PARENT NAME

OF ALKANE

NUMBER OF

CARBON ATOMS

PARENT NAME

OF ALKANE

If there is a competition between two chains of equal length, then choose the chain with the

greater number of substituents. Substituents are branches connected to the par ent chain:

Correct

(3 substituents)

Incorrect

(2 substituents)

T e term “cyclo” is used to indicate the presence of a ring in the structure of an alkane. For

example, these compounds are called cycloalkanes:

Cyclopropane Cyclobutane Cyclopentane

Klein3e_ch04_132-180_LR_v3.1.indd 134 11/07/16 12:05 PM

Copy table – left side

Write out Question only (“Learn the

skill”) We will practice the skill in class together

after lecture

Practice with Skillbuilder 4.1 (p.135)

Rule #2: Identify and name the

substituents

Substituents end in yl

instead of ane.

Copy table

A ring can be either a parent chain or a

substituent depending on the number of

carbons



after lecture


For substituents with complex branches

1. Number the longest carbon chain WITHIN the substituent. Start with the carbon attached to the parent chain

2. Name the substituent (in this case butyl)

3. Name and Number the substituent’s side group (in this case 2-methyl)

The name of the substituent is (2-methylbutyl)

1 2 3

4

Some branched substituents have

common names

Two types of propyl groups

Three types of butyl groups



after lecture


Rule #3: Carbons in the parent chain

have to be numbered

2-methylpentane means there is a

methyl group on carbon #2 of the

pentane chain

Guidelines to follow when numbering the parent

chain

1. If ONE substituent is present,

number the parent chain so that the

substituent has the lowest number

possible

2.When multiple substituents are

present, number the parent chain to

give the first substituent the lowest

number possible number

3. If there is a tie, then number the

parent chain so that the second

substituent gets the lowest number

possible

4. If there is no other tie-breaker, then

assign the lowest number alphabetically

The same rules apply for cycloalkanes

To assemble the complete name:

Put the # and name of each substituent

before the parent chain name, in

alphabetical order

A prefix is used (di, tri, tetra, penta, etc.)

if multiple substituents are identical.

note: “di” or “tri” is ignored when

alphabetizing the substituents

IUPAC Rules - Summary

1. Identify the parent chain

2. Identify and Name the substituents

3. Number the parent chain; assign a locant to each substituent

4. List the numbered substituents before the parent name in alphabetical order

Following the rules, we can

name the following compound:

Parent name:

cyclohexane

Substituents:

1-tert-butyl

2-ethyl

4-methyl

4-methyl 4,4-dimethyl

The name is……. 1-tert-butyl-2-ethyl-4,4-dimethylcyclohexane



after lecture


Bicyclic compound contains two

fused rings.

To name a bicyclic compound,

include the prefix bicyclo- in front of

the parent name

The two carbons where the

rings are fused are bridgehead

carbons

There are three “paths (carbon

chains)” connecting the

bridgeheads.

4.2 Nomenclature of Alkanes 143

T e problem is that this parent is not specif c enough. To illustrate this, consider the following two

compounds, both of which are called bicycloheptane:

Both compounds consist of two fused rings and seven carbon atoms. Yet, the compounds are

clearly dif erent, which means that the name of the parent needs to contain more information.

Specif cally, it must indicate the way in which the rings are constructed. In order to do this, we

must identify the two bridgeheads, which are the two carbon atoms where the rings are fused

together:

Bridgehead

Bridgehead

T ere are three dif erent paths connecting these two bridgeheads. For each path, count the number

of carbon atoms, excluding the bridgeheads themselves. In the compound above, one path has two

carbon atoms, another path has two carbon atoms, and the third (shortest path) has only one carbon

atom. T ese three numbers, ordered from largest to smallest, [2.2.1], are then placed in the middle

of the parent, surrounded by brackets:

Bicyclo[2.2.1]heptane

T ese numbers provide the necessary specif city to dif erentiate the compounds shown earlier:

Bicyclo[2.2.1]heptan eBicyclo[3.1.1]heptan e

If a substituent is present, the parent must be numbered properly in order to assign the correct

locant to the substituent. To number the parent, start at one of the bridgeheads and begin numbering

along the longest path, then go to the second longest path, and f nally go along the shortest path. For

example, consider the following bicyclic system:

CH3

12

3

45

6

7

8

In this example, the methyl substituent did not get a low number. In fact, it got the highest number

possible because of its location. Specif cally, it is on the shortest path connecting the bridgeheads.

Regardless of the position of substituents, the parent must be numbered beginning with the longest

path f rst. T e only choice is which bridgehead will be counted as C1; for example:

Correct

12

34

56

7

8

CH3

Incorrect

12

3

45

6

7

8

CH3

Either way, the numbers begin along the longest path. However, we must start numbering at the

bridgehead that gives the substituent the lowest possible number. In the example above, the correct

path places the substituent at C6 rather than at C7, so this compound is 6- methylbicyclo[3.2.1]

octane.

Klein3e_ch04_132-180_LR_v3.1.indd 143 11/07/16 12:05 PM

Count the carbons from the longest path

(carbon chain) to the smallest path

(carbon chain).

Note that the bridgehead carbons

should be the first carbons numbered

and the peak carbons (carbons

protruding from bridgehead carbons)

should be the last carbons numbered

1

1

1

2 1 1 1

3 2 2



after lecture


READ pg. 146-147

then take notes

CONSTITUTIONAL ISOMERS

Same number of atoms but

different connectivity of atoms

As the number of

carbon atoms

increases, the

number of

constitutional

isomers increases

When drawing the constitutional isomers of an alkane, make sure to avoid drawing the same isomer twice. As an example, consider C6H14, for which there are five constitutional isomers.me compound.

You can test if structures are the same in two ways:

1. Flip one of the molecules and rotate around its single bonds until it can be placed over the other molecule

2. Name them. If they have the same IUPAC name, they are the same compound

180˚ rotation along the C3 – C4 bond

would make it more obvious these two

compounds are the same

Following IUPAC rules for naming yields

the same name as well



after lecture


READ pg. 147-148

then take notes

Relative stability of isomers can be

determined by measuring heat of combustion

For an alkane it is the reaction of an alkane

and oxygen to produce CO2 and H2O

What do you notice about the ΔH of

combustion the branches the alkane has?

No branches 2 branches 4 branches

ΔHo is the change in enthalpy, associated with the complete combustion of 1 mole of the alkane in the presence of oxygen

For a combustion process, -ΔHo is called the heat of combustion

By comparing the heat of combustion of all constitutional isomers we can determine the most stable isomer = lowest amount of energy released (exothermic – heat given off during reaction) in combustion

Combustion can be conducted under experimental conditions using a device called a calorimeter

Branched alkanes are lower in energy (more stable) than straight-chain alkanes

We will study more about enthalpy in Ch 6

READ pg. 150-152

then take notes

Single bonds rotate, resulting in multiple 3-D shapes, called conformations

There are various ways to represent the 3-D shape of a compound

Some conformations are higher in energy, while others are lower in energy.

In order to draw and compare conformations, we will need to use a new kind of drawing—one specially designed for showing the conformation of a molecule.

Newman projections are ideal for comparing the relative stability of possible conformations resulting from single bond rotation (Some

This how Newman projections are drawn…

Begin rotating it about the vertical axis drawn in

gray so that all of the red H’s come out in front of

the page and all of the blue H’s go back behind

the page.

The second drawing (the sawhorse) represents a

snapshot after 45° of rotation, while the Newman

projection represents a snapshot after 90° of

rotation.

One carbon is directly in front of the other, and

each carbon atom has three H’s attached to it.

A Newman projection is the perspective of looking straight down a particular C-C bond

The point at the center of the drawing represents the front carbon atom, while the circle represents the back carbon.

All the hydrogens in red are coming out of the page and all the blue hydrogens are going to the back of the page.

We will use Newman projections extensively throughout the rest of this chapter, so it is important to master both drawing and reading them.



after lecture


READ pg. 152-154

then take notes

These two hydrogen atoms

appear to be separated by an

angle of 60°. (360/6)

The angle between atoms on

adjacent carbons is called a

dihedral angle or torsional

angle. It is 60° in the molecule

below

The dihedral angle changes as

the C-C bond rotates. Which

makes it so that there is an

infinite amount of conformations

However, we only care about the staggered conformation (lowest in energy and most stable) and the eclipsed conformation (highest in energy and least stable)

The difference in energy between these conformations is due to torsional strain. Here, the difference in energy is 12 kJ/mol

All staggered conformations are

degenerate (have the same amount of

energy) and all eclipsed conformations

are degenerate

It’s possible the eclipsed conformation is 12 kJ/mol less stable because of electron pair repulsion between the eclipsing bonds (4 kJ/mol for each eclipsing interaction)

With a difference of 12 kJ/mol in stability, at room temperature, 99% of the molecules will be in the staggered conformation

The difference in energy can also be

rationalized by the presence of stabilizing

interactions in the staggered conformation

A filled, bonding MO

has side-on-side

overlap with an

empty anti-bonding

MO.

The analysis of torsional strain for

propane (below) is similar to ethane

The barrier to rotation for propane

is 14 kJ/mol, which is 2 kJ/mol more

than for ethane

If each H-----H eclipsing interaction

costs 4 kJ/mol of stability, that total

can be subtracted from the total 14

kJ/mol to calculate the contribution of

a CH3-----H eclipsing interaction

READ pg. 154 -156

then take notes

The analysis of torsional strain for

butane shows more variation

Note that there

are multiple

staggered

conformations

and multiple

eclipsed

conformations

The three highest energy conformations are the eclipsed conformations, while the three lowest energy conformations are the staggered conformations.

In this way, the energy diagram above is similar to the energy diagrams of ethane and propane.

But in the case of butane, notice that one eclipsed conformation (where dihedral angle = 0) is higher in energy than the other two eclipsed conformations.

In other words, the three eclipsed conformations are not degenerate.

Similarly, one staggered conformation (where dihedral angle = 180°) is lower in energy than the other two staggered conformations.

Clearly, we need to compare the staggered conformations to each other, and we need to compare the eclipsed conformations to each

Let’s begin with the three staggered

conformations.

The conformation with a dihedral angle of

180° is called the anti conformation, and

it represents the lowest energy

conformation of butane.

The other two staggered conformations

are 3.8 kJ/mol higher in energy than the

anti conformation. Why? We can more

easily see the answer to this question by

drawing Newman projections of all three

staggered Conformations

In the anti conformation, the methyl groups achieve

maximum separation from each other.

In the other two conformations, the methyl groups are

closer to each other. Their electron clouds are repelling

each other (trying to occupy the same region of space),

causing an increase in energy of 3.8 kJ/mol.

This unfavorable interaction, called a gauche

interaction, is a type of steric interaction, and it is

different from the concept of torsional strain.

The two conformations above that exhibit this

interaction are called gauche conformations, and they

are degenerate

Now let’s turn our attention to the three eclipsed

conformations.

One eclipsed conformation is higher in energy than the

other two. Why? In the highest energy conformation,

the methyl groups are eclipsing each other.

Two of the eclipsed conformations of butane are

degenerate.

Each CH3-----CH3 eclipsing interaction accounts for 11

kJ/mol of energy (torsional and steric strain).

In each case, there is one pair of eclipsing H’s and two

pairs of eclipsing H/CH3. We have all the information

necessary to calculate the energy of these

conformations. We know that eclipsing H’s are 4

kJ/mol, and each set of eclipsing H/CH3 is 6 kJ/mol.

Therefore, we calculate a total energy cost of 16 kJ/mol

To summarize, we have seen just a few numbers that

can be helpful in analyzing energy costs.

With these numbers, it is possible to analyze an

eclipsed conformation or a staggered conformation and

determine the energy cost associated with each

conformation.



after lecture


READ pg. 158 -160

then take notes

Towards the end of the 19th century Adolph

von Baeyer proposed a theory describing

cycloalkanes in terms of angle strain - the

increased in energy associated with a bond

angle that has deviated from the preferred

angle if 109.5

Ideal bond angles for sp3 hybridized carbon is

109.5˚

If cycloalkanes were flat, each carbon in the ring

would experience angle strain.

Also, if a ring was flat, then all the C-C bonds would be in eclipsing conformation … causing considerable torsional strain.

However, since Baeyer theory was based on the assumption that cycloalkanes are flat (planar) it did not hold because the carbon ring can position themselves in 3D space to achieve a staggered conformation

The combustion data for cycloalkanes shows

that a 6-member ring is the most stable ring

size (it is lowest in energy per CH2 group)

Cyclopropane

Two main factors contributing to its high

energy:

angle strain (from small bond angles)

torsional strain (from eclipsing H’s)

As a result of the large

amount of strain makes

the 3-membered rings

highly reactive and

susceptible to ring-

opening reactions

Cyclobutane Cyclobutane has less angle strain than

cyclopropane.

1. Angle strain bond angles of 88-90°

2. Has more torsional strain, because there are

four sets of eclipsing H’s rather than just three.

To alleviate some of this

additional torsional strain,

cyclobutane can adopt a

slightly puckered

conformation (has less

torsional strain than a flat

conformation)

Cyclopentane Cyclopentane has much less angle strain than

cyclobutane or cyclopropane. It can also reduce

much of its torsional strain by adopting the following

conformation.

Cyclopentane

1. Very little angle strain - bond angles are nearly

109.5˚

2. Slight torsional strain – adopts an envelope

conformation to avoid most of it

READ pg. 161-162

then take notes

Cyclohexane can adopt many conformations

We will explore two conformations:

Chair conformation

Boat conformation

Both conformations possess very little angle strain

The significant difference between them can be seen when comparing torsional strain.

Chair conformation:

No angle strain – bond angles are 109.5°

No torsional strain - all adjacent C-H bonds are

staggered (none are eclipsed as it can be seen with a

Newman projection)

The other possible conformations of cyclohexane

have some amount of angle and/or torsional

strain (i.e. ring strain)

Boat Conformation:

Has two sources of torsional strain.

1. Many of the H’s are eclipsed

2. H’s on either side of the ring experience steric

interactions called flagpole interactions.

The boat can alleviate some of this torsional strain by

twisting (very much the way cyclobutane puckers to

alleviate some of its torsional strain), giving a

conformation called a twist boat.

Cyclohexane can adopt many different

conformations, but the most important is

the chair conformation - most stable

(lowest energy).

There are actually two different chair

conformations that rapidly interchange via

a pathway that can pass through many

different conformations, including a high-

energy half-chair conformation, as well as

twist boat and boat conformations.

Energy diagram summarizing the relative energy levels of the various conformations of cyclohexane.

The lowest energy conformations are the two chair conformations, and therefore, cyclohexane will spend the majority of its time in a chair conformation.

READ pg. 162 -164

then take notes

But first step we must master drawing them….

The following procedure outlines a step-by-step

method for drawing the skeleton of a chair

conformation precisely for cyclohexane:

When you are finished drawing the chair, it

should contain 3 sets of parallel lines.

If you chair does not contain 3 sets of parallel

lines, then it has been drawn incorrectly.

Each carbon in the ring has two substituents:

1. Axial position - parallel to a vertical axis

passing through the center of the ring.

2. Equatorial position - positioned approximately

along the equator of the ring.

In order to draw a substituted cyclohexane, we

must first practice drawing all axial and equatorial

positions properly.

Let’s practice…Skillbuilder 4.10

Draw all axial and all equatorial positions on a chair

conformation of cyclohexane.

Let’s begin with the axial positions, as they are easier to

draw.

Begin at the right side of the V and draw a vertical line

pointing up. Then, go around the ring, drawing vertical

lines, alternating in direction (up, down, up, etc.).

These are the six axial positions. All six lines are vertical.

Now let’s draw the six equatorial positions.

The equatorial positions are more difficult to draw

properly, but mistakes can be avoided in the following

way.

We saw earlier that a properly drawn chair skeleton is

composed of three pairs of parallel lines.

Now we will use these pairs of parallel lines to draw the

equatorial positions (blue lines). In between each pair of

red lines, we draw two equatorial groups that are parallel

to (but not directly touching) the red lines:

READ pg. 164 -166

then take notes

Drawing Both Chair Conformations

Consider a ring containing only one

substituent.

Two possible chair conformations can be

drawn:

The substituent can be in an axial position or in

an equatorial position.

These two possibilities represent two different

conformations that are in equilibrium with each

other:

Ring flip

The term “ring flip” is used to describe the conversion of one chair conformation into the other.

This process is not accomplished by simply flipping the molecule like a pancake.

Rather, a ring flip is a conformational change that is accomplished only through a rotation of all C−C single bonds.

This can be seen with a Newman projection

Axial substituents become equatorial and vice versa.

Practice with Skillbuilder 4.11 (p.164) – Write it out

Draw both chair conformations of bromocyclohexane:

STEP 1: Draw a chair conformation.

STEP 2: Place the substituent.

Continue…

When two chair conformations are in

equilibrium, the lower energy conformation

will be favored.

Consider methylcyclohexane.

At room temperature, 95% of the

molecules will be in the chair conformation

that has the methyl group (Me) in an

equatorial position. This must therefore be

the lower energy conformation, but why?

When the substituent is in an axial position, there are

steric interactions with the other axial H’s on the

same side of the ring.

The substituent’s electron cloud is trying to occupy

the same region of space as the H’s that are

highlighted, causing steric interactions.

These interactions are called 1,3 – diaxial

interactions, where the numbers “1,3” describe the

distance between the substituent and each of the H’s.

When the chair conformation is drawn in a Newman

projection, it becomes clear that most 1,3-diaxial

interactions are nothing more than gauche

interactions.

The presence of 1,3-diaxial interactions

The steric strain from a substituent being in the axial

position is the result of 1,3-diaxial interactions, which

are are actually gauche interactions

Causes the chair conformation to be higher in

energy when the substituent is in an axial position.

In contrast, when the substituent is in an equatorial

position, these 1,3-diaxial (gauche) interactions are

not present .

For this reason, the equilibrium between

the two chair conformations will generally

favor the conformation with the equatorial

substituent.

However, it all depends on the size of the

substituent.

Larger groups will experience greater

steric interactions, and the equilibrium will

more strongly favor the equatorial

substituent.

READ pg. 166 -170

then take notes

With multiple substituents, solid or

dashed wedges are used to show

positioning of the groups on the ring

Chlorine atom is on a wedge, which

means that it is coming out of the page: it

is UP. The methyl group is on a dash,

which means that it is below the ring, or

DOWN.

Cl atom is above the ring (UP) in both chair conformations, and the methyl group is below the ring (DOWN) in both chair conformations.

The configuration (i.e., UP or DOWN) does not change during a ring flip.

It is true that the chlorine atom occupies an axial position in one conformation and an equatorial position in the other conformation, but a ring flip does not change configuration.

The chlorine atom must be UP in both chair conformations and the methyl group must be DOWN in both chair conformations.


Draw both chair conformations of the following

compound:

STEP 1

Determine the location and configuration of each

substituent.

FYI - It does not matter where the numbers are placed;

these numbers are just tools used to compare

positions in the original drawing and in the chair

conformation to ensure that all substituents are placed

correctly.


STEP 2

Place the substituents on the first chair using the

information from step 1.

STEP 3

Place the substituents on the second chair using the

information from step 1.


Therefore, the two chair conformations of this

compound are:

Comparing the Stability of Chair Conformations:

Lets consider the following

In the first conformation, both groups are equatorial.

In the second conformation, both groups are axial.

In the previous section, we saw that chair conformations will be lower in energy when substituents occupy equatorial positions (avoiding 1,3-diaxial interactions).

Therefore, the first chair will certainly be more stable.

In some cases, two groups might be in

competition with each other. For example,

consider the following compound:

In this example, neither conformation has two

equatorial substituents.

In the first conformation, the chlorine is

equatorial, but the ethyl group is axial.

In the second conformation, the ethyl group is

equatorial, but the chlorine is axial.

In a situation like this, we must decide which group exhibits a greater preference for being equatorial: the chlorine atom or the ethyl group. To do this, we use the numbers from Table 4.8:

Both conformations will exhibit 1,3-diaxial interactions, but these interactions are less pronounced in the second conformation.

The energy cost of having a chlorine atom in an axial position is lower than the energy cost of having an ethyl group in an axial position.

Therefore, the second conformation is lower in energy.

https://jigsaw.vitalsource.com/books/9781119351603/epub/OPS/ch04.xhtml#ch04-tbl-0008


Draw the more stable chair conformation of the

following compound:

STEP 1

Determine the location and configuration of

each substituent.


STEP 2

Draw both chair conformations.


STEP 3

Assess the energy cost of each axial group.

• In the first conformation, there is one ethyl group

in an axial position. According to Table 4.8, the

energy cost associated with an axial ethyl group

is 8.0 kJ/mol.

• In the second conformation, two groups are in

axial positions: a methyl group and a chlorine

atom. According to Table 4.8, the total energy

cost is 7.6 kJ/mol + 2.0 kJ/mol = 9.6 kJ/mol.

• Energy cost is lower for the first conformation

(with an axial ethyl group). The first conformation

is therefore lower in energy (more stable).



READ pg. 170 -171

then take notes

When naming a disubstituted

cycloalkane, use the prefix cis when

there are two groups on the same side of

the ring, and trans when two

substituents are on opposite sides of a

ring

The drawings above are Haworth projections

(as seen in Section 2.6) and are used to clearly

identify which groups are above the ring and

which groups are below the ring.

https://jigsaw.vitalsource.com/books/9781119351603/epub/OPS/ch02.xhtml#ch02sec6

Each compound above is better represented as an equilibrium between two chair conformations.

cis-1,2-Dimethylcyclohexane and trans-1,2-dimethylcyclohexane are stereoisomers (Starts Ch 5).

They are different compounds with different physical properties, and they cannot be interconverted via a conformational change.

trans-1,2-Dimethylcyclohexane is more stable, because it can adopt a chair conformation in which both methyl groups occupy equatorial positions.

Each compound exists as two

equilibrating chairs, spending more time in

the more stable chair conformation

This is the lowest

energy conformation

for the cis isomer

This is the lowest

energy conformation

for the trans isomer

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