Bis(hydroxyethyl) terephthalate

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

(Q)SAR

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification

Justification for type of information:

Based on the effects of various electron withdrawing groups, the pKa of BHET was predicted

Qualifier:

no guideline followed

Principles of method if other than guideline:

Based on the effects of various electron withdrawing groups, the pKa of BHET was predicted

GLP compliance:

no

Specific details on test material used for the study:

The pKa was predicted based on chemical structure.

Dissociating properties:

yes

No.:

#1

pKa:

> 12.24

Temp.:

22 °C

According to Streitwieser et al. (1998) alcohols can exhibit acidic properties but aliphatic alcohols are generally less acidic than water with pKa values often greater than 15.  In general, adjacent electron withdrawing groups can increase the acidity of an alcohol because the conjugate base that forms will be more stable as the negative charge will be pulled towards the electron withdrawing group via inductive effect.

However, it is known that inductive effects diminish rapidly as one moves away from the source along a carbon chain, and are practically insignificant beyond the second carbon atom (Finar, 1976), so the presence of the electron-withdrawing benzoate group on the second carbon is not expected to exert any significant effect on the acidity of the alcohol moieties in the notified substance.

Ballinger & Long (1959) present the effects of various electron withdrawing groups on the pKa of methanol, i.e., when attached to the first carbon of the alcohol, as summarised in Table 1.

Table 1                  Substituent Effects on Dissociation of RCH2OH ↔ RCH2O– + H+

R–	pKa	R–	pKa
H– CH2=CH– HOCH2– CH3OCH2– CH2Cl–	15.5 15.5 15.1 14.8 14.31	CH≡C– CHCl2– CHF2CF2– CF3– CCl3–	13.55 12.89 12.74 12.37 12.24

 

From Table 1, it can be seen that the pKa of the alcohol group decreases (i.e., its acidity increases) as the strength of the electron-withdrawing group increases.  Given that the trichloromethyl substituent, which is expected to have similar inductive effects as the benzoate group, has been shown to lower the alcohol pKa to 12.24 when attached to the first carbon, it is reasonable to expect that presence of the benzoate group on the second carbon of the alcohol groups in the notified substance will not reduce their pKa values as much, and they will therefore be > 12.24.

Conclusions:

Based on the evidence presented above, the dissociation constants of the notified substance are predicted to be pKa > 12.24, and the notified substance can confidently be predicted to exist essentially entirely in neutral, undissociated form throughout the environmentally relevant range of pH 4 to 9.

Description of key information

Based on the evidence presented above, the dissociation constants of the notified substance are predicted to be pKa > 12.24, and the notified substance can confidently be predicted to exist essentially entirely in neutral, undissociated form throughout the environmentally relevant range of pH 4 to 9.