INTRODUCTION TO THE ARYL HALIDES (HALOGENOARENES)

موضوع: INTRODUCTION TO THE ARYL HALIDES (HALOGENOARENES) 2/11/2008, 05:44

INTRODUCTION TO THE ARYL HALIDES (HALOGENOARENES) This page looks at the structure and physical properties of three simple aryl halides - chlorobenzene, bromobenzene and iodobenzene. An aryl halide has a halogen atom attached directly to a benzene ring. The structure of chlorobenzene We'll look in some detail at the structure of chlorobenzene. Bromobenzene and iodobenzene are just the same. The simplest way to draw the structure of chlorobenzene is: . . . but to understand chlorobenzene properly, you need to dig a bit deeper than this.

There is an interaction between the delocalised electrons in the benzene ring and one of the lone pairs on the chlorine atom. This overlaps with the delocalised ring electron system . . . . . . giving a structure rather like this: This delocalisation is by no means complete, but it does have a significant effect on the properties of both the carbon-chlorine bond and the polarity of the molecule. The delocalisation introduces some extra bonding between the carbon and the chlorine, making the bond stronger. This has a major effect on the reactions of compounds like chlorobenzene. There is also some movement of electrons away from the chlorine towards the ring. Chlorine is quite electronegative and usually draws electrons in the carbon-chlorine bond towards itself. In this case, this is offset to some extent by the movement of electrons back towards the ring in the delocalisation. The molecule is less polar than you would otherwise have expected. Physical properties Boiling points Chlorobenzene, bromobenzene and iodobenzene are all oily liquids. The boiling points increase as the halogen atom gets bigger. <table class=MsoNormalTable style="mso-cellspacing: 1.5pt; mso-yfti-tbllook: 1184; mso-padding-alt: 3.75pt 3.75pt 3.75pt 3.75pt" cellPadding=0 border=1><tr style="mso-yfti-irow: 0; mso-yfti-firstrow: yes"><td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"></TD> <td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> boiling point (°C) </TD></TR> <tr style="mso-yfti-irow: 1"><td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> C₆H₅Cl</TD> <td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">[center]132 </TD></TR> <tr style="mso-yfti-irow: 2"><td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> C₆H₅Br</TD> <td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">[center]156 </TD></TR> <tr style="mso-yfti-irow: 3; mso-yfti-lastrow: yes"><td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> C₆H₅I</TD> <td style="BORDER-RIGHT: #f0f0f0; PADDING-RIGHT: 3.75pt; BORDER-TOP: #f0f0f0; PADDING-LEFT: 3.75pt; PADDING-BOTTOM: 3.75pt; BORDER-LEFT: #f0f0f0; PADDING-TOP: 3.75pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> 189 </TD></TR></TABLE>

The main attractions between the molecules will be van der Waals dispersion forces. These increase as the number of electrons in the molecule increases. This is the reason that the boiling points increase as the halogen atom gets bigger. There will also be permanent dipole-dipole attractions involved in the chlorobenzene and bromobenzene, but very little in the iodobenzene. Iodine has much the same electronegativity as carbon. These dipole-dipole attractions must be very unimportant relative to the dispersion forces because the most polar molecule (the chlorobenzene) has the lowest boiling point of the three. Solubility in water The aryl halides are insoluble in water. They are denser than water and form a separate lower layer. The molecules are quite large compared with a water molecule. In order for chlorobenzene to dissolve it would have to break lots of existing hydrogen bonds between water molecules. You also have to break the quite strong van der Waals dispersion forces between chlorobenzene molecules. Both of these cost energy. The only new forces between the chlorobenzene and the water would be van der Waals dispersion forces. These aren't as strong as hydrogen bonds (or the original dispersion forces in the chlorobenzene), and so you wouldn't get much energy released when they form. It simply isn't energetically profitable for chlorobenzene (and the others) to dissolve in water.

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Thank u sir.A....about these info. ,,in fact i congratulate me,becoz u joint us

Thanx a lot :) and don't worry I'll upload other subject as soon as possible..

BEST WISHES

زائر

merçi beaucoup
dr.ANATOMY 4 these infromation,& welcome 2 u in our montada, & we r waiting from u a lot of participation.
Sallam>>>>>>>>
but, can u tell me y chlorine is donate it's pair of electrons 2 c in benzen ring,althouth chlorine is more electronegativity than c in benzen ring? who knows???
also, who can tell me y chlorine is deactivating group that oriented 2 o & p position,
but,OH GROUP is also oriented 2 o & p , but it is activating group?
or y chlorine is weak activating group????? who knows???
I KNOW THE ANSWER ((MAY BE)) hahhahahahahah>

Hey .... thanx a lot ... and about ur question... I'll search 4 the answer and I'll inform u as soon as possible....

BEST WISHES :)

زائر

Ok ya man im gana wait u.but i wana from u 2 try 2 get the answers from ur head. & then take the answers from NET. & make a comparison between them.
Sallam

HeY again..... I have thought a lot of ur question and search 4 an answer on the net "I put ur question on Yahoo Answers " But I didn't find an answer!!!!
If u can re-write the question using another way that will be better I think..
Or go and ask Dr:Z... he'll give u the perfect ANSWER " I guess "
And if u have the answer just write it !!!! I have no time to think or such things :lol!: BEST WISHES

زائر

hahaha.............. I know that u can't find the answers in NET By any way. I don't need Dr. Z 2 tell me the answers,bcuz I know them,
I wana from u 2 think,don't b lazy ya man.
about writing the answers, it is difficut here 2 show them very well, please don't think that((im selfish ))>>
keep in touch
Sallam

No man I have never thougth that u r selfish or something !!!!!
And I really love when someone challenge me to find an answer J.. and I'm really not lazy but I'm busy these days ( not 4 studying but I have other things ) …. Anyway, I'll keep thinking "searching" of ur question till I find answers as soon as possible just wait …….. thanx a lot 4 ur passing
BEST WISHES J

زائر

ok ya man>>>>>>>>>>>>>>>
may ALLAH prosper u.

( God willing ) ..... and thanx again 4 ur passing :lol!:

BEST WISHES :)

مرسيه بكوه

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